 -- The Senate voted 51-50 on Tuesday to move forward with a debate on health care reform, even though it was not clear what measure the body will be considering. Vice President Mike Pence cast the tie-breaking vote.

Susan Collins of Maine and Lisa Murkowski of Alaska were the only two Republicans who joined Democrats in voting no on the motion to proceed.

Before the voting began, protesters in the Senate's public gallery chanted "Kill the bill" and "Shame! Shame!"

Sen. John McCain, R-Ariz., who was diagnosed with brain cancer last week, returned to the Senate floor to vote in favor of moving the debate forward. His appearance was met with a standing ovation.

"I voted for the motion to proceed to allow debate to continue and amendments be offered," McCain said. "I will not vote for this bill as it is today. It's a shell of a bill right now. We all know that."

The Senate GOP has been dealt several setbacks, including not having enough votes for its original plan to repeal and replace aspects of Obamacare or for a straight repeal, but President Trump has pushed for Republican lawmakers to make progress on one of his legislative priorities.

Senate Majority Leader Mitch McConnell said the debate on health care in the chamber will be "an open amendment process."

"This is just the beginning. We're not out here to spike the football. This is the long way," McConnell said after the vote. "But we'll finish at the end of the week, hopefully with a measure that can either go to the House and be taken up or go to conference" — in which differences between the chambers' bills are resolved. "And we're pleased to have been able to take the first step, and that's the direction for today."

In a statement released today, Trump said he applauded senators for "taking a giant step to end the Obamacare nightmare."

"As this vote shows, inaction is not an option, and now the legislative process can move forward as intended to produce a bill that lowers costs and increases options for all Americans," the statement said. "The Senate must now pass a bill and get it to my desk so we can finally end the Obamacare disaster once and for all."

Trump thanked McCain on Twitter for returning to D.C. "for such a vital vote."