Sen. John McCain expected to return to Arizona to start cancer treatment Monday

US Republican Senator John McCain asks questions to Attorney General Jeff Sessions (unseen) during a US Senate Select Committee on Intelligence hearing on Capitol Hill in Washington, DC, June 13, 2017. US Attorney General Jeff Sessions vehemently denied Tuesday that he colluded with an alleged Russian bid to tilt the 2016 presidential election in Donald Trump's favor. / AFP PHOTO / SAUL LOEB

There’s a reason why Sen. John McCain was eager to pivot to the annual defense spending bill in the early hours of the morning right after he had killed the Senate’s Obamacare repeal effort: Three congressional sources told CNN the Arizona Republican is planning to return to Arizona to start cancer treatment Monday.

That may also help explain why Majority Leader Mitch McConnell appeared willing to let McCain try to proceed to the the National Defense Authorization Act, even after he thwarted the GOP’s plan to repeal and replace the Affordable Care Act in the early hours of Friday morning.

McCain is chairman of the armed services committee, and he leads the annual defense bill that authorizes Pentagon spending and sets military policy. Following his brain cancer diagnosis earlier this month, it was unclear whether McCain would be in position to lead floor debate on the measure due to his possible treatment schedule.

After his dramatic return to the Senate this week, McCain had hoped the Senate could quickly debate and pass the defense authorization bill once health care was completed. Senate Democrats said they would be willing — if Republicans dropped the repeal bid.

So after the health care vote failed, McConnell tried to get unanimous consent to proceed to the defense bill early Friday morning, but Sen. Rand Paul objected, as he wanted votes on his amendments on war authorization and indefinite detention.

“Sen. Rand Paul requested two bipartisan amendments, one on ending indefinite detention and one on AUMFs,” Paul spokesman Sergio Gor said in a statement. “He looks forward to working with leadership and the committee to get this done soon.”

The Senate is now expected to push back McCain’s defense bill until September, but it’s not known whether McCain will be back by then.