Michelle Obama addresses Democratic National Convention Tuesday

CHARLOTTE, North Carolina (CNN) -- A Democratic National Convention vital to President Barack Obama's re-election bid opened Tuesday with praise for his battles on behalf of minority rights and the middle class, as well as plentiful attacks on Republican challenger Mitt Romney.

Facing a tight race in November and Republican attacks that they have made things worse while in power, Democratic organizers want their three-day convention to emphasize the tough decisions Obama has made and the additional steps needed to bolster the middle class.

The more than 5,500 Democratic delegates adopted a party platform that emphasizes middle-class opportunity while differing sharply on key issues with the Republican version endorsed at last week's GOP convention.

Then speech after speech lambasted Romney and Republicans, accusing them of being out of touch and politically divisive at a time requiring national unity to confront high unemployment, a sluggish economy and mounting federal deficits and debt.

Seeking to further strengthen Obama's advantage with women, Hispanic Americans and young voters, the Democratic speakers hailed the president for promoting health care reforms, supporting gay marriage, and ending deportations of some young illegal immigrants.

First lady Michelle Obama offered a personal perspective on why her husband should be re-elected, telling the convention that the same values she fell in love with guide him each day in the White House.

"In the end, for Barack, these issues aren't political -- they're personal," Mrs. Obama said, adding: "Barack knows the American Dream because he's lived it, and he wants everyone, everyone in this country to have the same opportunity, no matter who we are, or where we're from, or what we look like, or who we love."

Health and Human Services Secretary Kathleen Sebelius defended the 2010 health care law despised by Republicans, saying the changes that prevent insurance companies from denying coverage for pre-existing conditions and allow parents to keep their children on family policies up to age 26 were "what change looks like."

She said Romney and GOP running mate Rep. Paul Ryan want to repeal the health care law and effectively end the Medicare program for senior citizens by making it a voucher program.

"Mitt Romney, quite simply, doesn't get it," declared San Antonio Mayor Julian Castro in his keynote address, later adding: "We know that in our free-market economy, some will prosper more than others. What we don't accept is the idea that some folks won't even get a chance."

Romney and Republicans "are perfectly comfortable with that America -- in fact, that's exactly what they're promising us," Castro said.

Massachusetts Gov. Deval Patrick earlier challenged Obama supporters to be more forceful in supporting the president's record in the face of unrelenting Republican attempts to discredit the administration's accomplishments, such as ending the Iraq war and delivering "the security of health care to every single American in every single corner of the country."

"It's time for Democrats to grow a backbone and stand up for what we believe," Patrick said to cheers..

Other speakers mocked Romney's overseas financial holdings -- a Swiss bank account, a Cayman Islands tax shelter -- revealed in the two years of tax returns the former Massachusetts governor has released so far. Senate Majority Leader Harry Reid challenged Romney to make public more of his tax records, citing the example of the 12 years of returns Romney's father, George, made public in his 1968 presidential run.

"Trust comes from transparency, and Mitt Romney comes up short on both," Reid said.

In a video played in the Time Warner Cable Arena where the convention opened, former President Jimmy Carter said Obama prioritized the middle class in making the difficult decisions required of the nation's highest office.

"He has done it all in the face of bitter, unyielding, in fact unprecedented partisan opposition," Carter said in reference to congressional Republicans.

The convention will conclude Thursday with Obama accepting his party's nomination exactly two months before the November 6 election. Former President Bill Clinton will address the gathering on Wednesday night.

Newark Mayor Corey Booker energized Tuesday's first session of the convention gaveled to order Tuesday afternoon with a rousing call for delegates to pass the party platform that represented "our fundamental national aspiration."

To applause and cheers, Booker said voters in November faced a choice between GOP policies offering either "a country of savage disparity favoring the fortunate few" or the Democratic goal of equal opportunity for all.

"This is our American mission," he added.

Later, Chicago Mayor Rahm Emanuel recalled the litany of crises that greeted the Obama administration when it assumed power in January 2009 -- a Wall Street meltdown, economic recession, wars in Iraq and Afghanistan, a collapsing auto industry.

"Each crisis was so deep, and so dangerous, any one of them would have defined another presidency," said Emanuel, who was Obama's chief of staff. "We faced a once-in-generation moment in American history, and fortunately, we have a once-in-a generation president."

Other speakers told how Obama's decision to bail out the struggling auto industry saved vital jobs and kept a mainstay of the U.S. economy afloat. The goal was to contrast a politically unpopular decision -- spending tax dollars to help a private industry -- with Romney's call at the time to let Detroit go bankrupt. Romney now says he advocated a managed bankruptcy similar to the eventual result under Obama.

Senior campaign officials told CNN that the convention has three main objectives -- to outline the clear choice facing voters, to highlight Obama's leadership in championing necessary but politically unpopular steps such as health care reform and the auto industry bailout, and to present a detailed plan for creating jobs for the middle class.

The Democratic platform focuses on improving the economic situation for middle-class Americans, and also contains language endorsing same-sex marriage and abortion rights that directly contrasts with the Republican platform adopted last week. In a break from past conventions, the platform does not include the word "God" or label Jerusalem as the capital of Israel -- two more issues that differ from Romney and Republicans.

"It's going to be up to you. You'll make the choice," Obama told a campaign event Tuesday at Virginia's Norfolk State University, framing the election as a decision between starkly different visions for the country's direction.

Romney's campaign is focused on the question of whether Obama has made life better for Americans, arguing that continued high unemployment and a sluggish economic recovery from the 2008 recession show that White House policies have failed to deliver promised results.

The "are you better off" strategy was famously employed in 1980 by Ronald Reagan, who asked voters that question when running against the incumbent Carter at a time of national economic woes. Reagan went on to win, and the Romney campaign has repeatedly invoked his name this year while seeking to link Obama and Carter as failed leaders.

Ryan, the conservative House Budget Committee chairman, has led efforts to link Obama with the Carter legacy, telling a campaign stop on Tuesday in Westlake, Ohio, that "when it comes to jobs, President Obama makes the Jimmy Carter years look like the good old days."

"If we fired Jimmy Carter, then why would we rehire Barack Obama now?" Ryan said to cheers. He later told an Iowa event that the national debt reached $16 trillion on Tuesday, which he called "a serious threat to our economy."

Obama and Vice President Joe Biden have fought back against the GOP attack line, telling supporters that the nation is better off now than it was when their administration took office in January 2009.

In Norfolk on Tuesday, Obama criticized Romney for failing to detail his economic plans, calling them "retreads of the same old polices that have been sticking it to the middle class for years." He cited the health care reform law he championed and financial industry reforms that followed the Wall Street meltdown as examples of accomplishments under his leadership that Romney opposes and has pledged to repeal.

The back-and-forth between the campaigns is part of their competition on how the election gets framed in the minds of voters. Republicans want it to be a referendum on Obama's presidency, while Democrats seek a choice between differing political ideologies on the size and role of government.

In particular, Republicans seek to shrink the size of government and end chronic federal deficits and rising national debt through reduced spending, reforming entitlement programs such as Social Security, Medicare and Medicaid, and slashing taxes on businesses and individuals as a spur for economic growth.

Obama and Democrats argue that a deficit-reduction plan also needs additional revenue as part of the equation, and they propose allowing tax rates on income of more than $250,000 for families and $200,000 for individuals to return to higher levels from the 1990s.

Republicans oppose any kind of tax increase, and the impasse over that issue has been the main impediment to a comprehensive deficit-reduction agreement during Obama's first term.

The race overall is very tight, with a new poll on Tuesday showing Romney received little bounce from last week's convention intended to introduce him to voters turning their attention to the presidential race.

The CNN/ORC International survey also indicates that less than 40% of registered voters said the GOP convention made them more likely to vote for Romney. At the same time, Romney got a slight bump in his favorable rating, and on being in touch with the middle class and women, although he still trails Obama on those two questions.

CNN's previous poll, released as the Republican convention got underway, indicated 49% of likely voters backed Obama, with 47% supporting Romney, a statistical tie. In the new survey, which was conducted after the GOP convention, both the president and Romney are at 48%.

"The Republican convention had at best a mild effect on the presidential race, and from a statistical viewpoint, no effect at all," said CNN Polling Director Keating Holland. "Demographically, Romney's overall one-point bounce masks some movement among subgroups and suggests that Romney's pitch to some groups may have worked but at the expense of turning off another group of voters."

While party platforms generally matter little to voters, the contrasts between the versions from this year's conventions showed the Republican adherence to traditional conservative positions while Democrats adopted more mainstream stances generally backed by American public opinion.

The Democratic platform omits a clause from the party's 2008 version proclaiming Jerusalem as Israel's capital, which drew immediate criticism from Romney.

"Four years of President Obama's repeated attempts to create distance between the United States and our cherished ally have led the Democratic Party to remove from their platform an unequivocal acknowledgment of a simple reality," the Republican nominee said in a statement. "As president, I will restore our relationship with Israel and stand shoulder to shoulder with our close ally."

Israelis consider Jerusalem the capital of their country, but Palestinians also claim rights to the city as the capital of a future independent state. The status of the city is designated for final negotiations between Israelis and Palestinians.

American policy has long been intentionally vague on the status of Jerusalem. A U.S. law passed in 1995 designates Jerusalem as the capital of Israel, and stipulates the American embassy should move to the city from Tel Aviv. The past three presidents, however, have signed waivers suspending the law, citing security and diplomatic concerns.

A Democratic National Committee spokesman said that Obama's stance on Jerusalem was consistent with previous administrations.

The omission of the world "God" from the Democratic platform represented a notable difference from Republicans, who mentioned God 10 times in their document approved at last week's GOP convention.

In 2008, Democrats wrote, "We need a government that stands up for the hopes, values, and interests of working people, and gives everyone willing to work hard the chance to make the most of their God-given potential."

Democrats did include a section in their 2012 platform specifically devoted to faith, writing faith "has always been a central part of the American story, and it has been a driving force of progress and justice throughout our history."

A Democratic official pointed out that 2008's reference to God was not specifically about faith, but rather about growing the middle class, and that the 2012 language specifically referring to faith was identical to 2008's document.

Meanwhile, Ryan continued to be hounded by factually challenged statements in his convention speech last week, coming out again Tuesday to defend a remark that appeared to blame Obama for the closure of a General Motors plant in Janesville, Wisconsin. An announcement that the plant would close preceded Obama's election.

"What I was saying is the president ought to be held to account for broken promises," Ryan said on NBC. "After the plant was shut down, he said he would lead efforts to restart the plant. It's still idle."

The Romney campaign reached out to reporters on Monday to point out that, during his Labor Day remarks in North Carolina, Ryan inaccurately described the number of bankruptcy filings under Carter when he was comparing his record to Obama.

Ryan also addressed his erroneous claim to have run a three-hour marathon in the past.

"I thought I ran an ordinary, kind of normal time, and I thought that was an ordinary time, until my brother showed me a three-hour marathon is crazy fast," Ryan responded when asked about the marathon claim in an interview with WTOL. "I ran a four-hour marathon. And so it's just the fact that I did this 22 years ago, and I forgot what my time was and that's what I thought it was."

CNN's Adam Aigner-Treworgy, Kevin Liptak, Peter Hamby, Sarah Aarthun and Paul Steinhauser contributed to this report.

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