Republican senators voted Tuesday to move forward on their seven-year pledge to repeal portions of Obamacare, with Vice President Mike Pence issuing the tie-breaking vote to unlock hours of debate to assemble details of a final healthcare bill.

The procedural motion was advanced along party lines by a 50-50 vote, with Pence breaking the tie. All 48 Democrats and two Republicans, Sens. Susan Collins of Maine and Lisa Murkowski of Alaska, voted no.

The vote was held open while senators waited for Sen. John McCain, R-Ariz., who was recently diagnosed with brain cancer, to arrive on the Senate floor.

The vote begins as much as 20 hours of debate on the House-passed healthcare bill, the American Health Care Act, and a new bill will be swapped in its place. A vote in favor of beginning debate does not necessarily indicate that a senator will support a final repeal bill.

Republicans are expected to debate repealing portions of Obamacare, including its taxes, and the individual mandate that requires people to buy insurance or pay a fine. The party has been hammering out details for several months, trying to bring various factions of the party together regarding changes to Medicaid and whether the Senate bill would adequately result in lower premiums. Centrists are concerned about the impact that repealing Obamacare would have on the law's Medicaid expansion and on people with pre-existing conditions, while conservatives say that the Senate bill, the Better Care Reconciliation Act, doesn't do enough to repeal Obamacare and bring down premiums.

The Congressional Budget Office estimated that 23 million more people would be uninsured by 2026 under the House-passed bill.

Before the vote began, about 20 protesters shouted "kill the bill" and, "Shame! Shame! Shame!" from the public viewing gallery after Senate Majority Leader Mitch McConnell, R-Ky., implored fellow Republicans to vote in favor of starting debate. They were escorted out before the vote began.

"We can't let this moment slip by," McConnell said in a floor speech ahead of the vote. "We've talked about this too long. We've wrestled with the issues, we've watched the consequences of the status quo. The people who sent us here expect us to begin this debate, to have the courage to tackle a tough issue."

Senators waited about 20 minutes for McCain to arrive and cast his affirmative vote, and several senators from both political parties were seen talking to Murkowski and Collins while they waited. Sen. Ron Johnson, who has complained about comments McConnell made to centrist holdouts about Medicaid changes in the Senate healthcare bill never taking effect, waited until McCain arrived to cast his "yes" vote. He spoke to McConnell for about 10 minutes on the Senate floor.

At the end of the vote, McCain spoke on the Senate floor and blasted the partisanship of the process. He warned that just because he was voting on the motion did not mean he would definitely support a final bill and said that if a bill failed, Democrats and Republicans should work together on solutions and hold hearings.

The current system is a mess, he said, adding: "What have we to lose by trying to work together to find a solution?"

Senate Minority Leader Chuck Schumer, D-N.Y., warned Republicans not to allow the bill to come to the floor because it would lead eventually to the full repeal of Obamacare.

"Turn back," Schumer urged the GOP. "We want to work with you. We know the Affordable Care Act is not perfect but we also know what you propose is much worse. We can work together to improve healthcare in our country. Turn back now before it's too late."

After the vote, Democrats required a reading of the 2015 repeal bill, which was passed by Congress but vetoed by former President Barack Obama, and is one of the options that Senate leaders are considering bringing up for a vote.