With President Donald Trump leaning on them from the White House, Republican leaders in the Senate are ready to push ahead with a procedural vote on Tuesday to begin debate on a House-passed GOP plan to overhaul the Obama health law, as Sen. John McCain (R-AZ) said he would return for the vote, even as top Republicans scrambled to figure out what changes they would offer to the health care bill in coming days.

With McCain back, Senate GOP leaders can only afford to lose two votes in the Senate, which would bring about a 50-50 tie, and then allow Vice President Mike Pence to cast the deciding vote to begin debate on health care legislation.

"Obamacare has been a disaster from the start," said Senate Majority Leader Mitch McConnell, who joined the President in pressing reluctant GOP Senators to vote to start the debate on Tuesday.

"The first vote we will take soon is on whether or not to begin the debate at all," McConnell said on the Senate floor, reminding Senators that they can't offer amendments if the bill isn't up for debate.

The return of Sen. McCain was not announced until late on Monday evening by his office, just nine days after he stunned Capitol Hill by announcing that he had been diagnosed with brain cancer, which delayed work on the issue.

But in a written statement issued by his office, McCain made clear that he was coming back for more than the health care vote.

"Senator McCain looks forward to returning to the United States Senate tomorrow to continue working on important legislation, including health care reform, the National Defense Authorization Act, and new sanctions on Russia, Iran and North Korea," the statement read.

While McCain will vote to start debate on health care, he has not guaranteed that he will support the GOP health care bill, as he has echoed the concerns of his state's Republican Governor, worried about how smaller increases in Medicaid money might impact his state in the future.

But even as Republicans moved toward this procedural vote - likely after members of both parties sit down for their regular lunch on Tuesday afternoon - no one knows yet what specific changes GOP leaders intend to offer for a vote, after the debate begins.

Technically, this is a vote to start debate on the bill approved by the House on health care - then, Republicans could start offering amendments on how to change the Obama health law, though the special rules of budget reconciliation will limit those amendments to matters dealing with spending and taxes.

As GOP leaders put the heat on Republican Senators to start that debate, they were getting help from the President, who framed the vote as a choice between keeping the status quo, or making an effort to change the Obama health law.

"Any Senator who votes against starting debate is telling America that you are fine with the Obamacare nightmare, which is what it is," Mr. Trump said at the White House.

"There's been enough talk - and no action," the President declared. "Now is the time for action."