WASHINGTON — The Senate launched into an uncertain legislative adventure Tuesday, as Majority Leader Mitch McConnell cajoled enough of his Republican colleagues to begin consideration of legislation to repeal Obamacare despite having no idea what the final bill will look like.

Without a single Democratic vote, 50 Republicans voted to begin Senate debate on a House-passed bill to repeal and replace Obamacare, a move Republicans have been promising voters for seven years. Sens. Lisa Murkowski of Alaska and Susan Collins of Maine both voted no on the "motion to proceed," leaving Vice President Pence to cast the tie-breaking vote to approve the motion and start the legislative debate.

The House bill — which President Trump has said is too “mean” — is unlikely to survive; McConnell quickly moved to replace it with a Senate-crafted version, but there were not enough votes to pass that either. Tuesday's vote opened the door for several days of amendments to get the Senate to a bill that can stand for a final vote by the end of the week. Under the rules McConnell is using, the final bill will only need the votes of 50 Republican senators to pass.

Sen. John McCain, R-Ariz., returning to the Senate after surgery last week to remove a blood clot that resulted in a brain cancer diagnosis, arrived late to the Senate floor and was greeted with a standing ovation from his colleagues. He voted in favor of debating the bill but then gave a speech excoriating the process the Senate is using to craft the legislation and suggesting it was likely to fail anyway.

But McConnell persisted. "This is just the beginning," he told reporters after the vote. He said he expects the Senate to pass a health care bill by the end of this week that can be sent to the House for approval or to a conference committee of House and Senate negotiators to craft a final compromise.

But an amendment that included the Better Care Reconciliation Act, the GOP replacement bill, failed Tuesday night — although it was not a surprise. This amendment was subject to Senate rules which required it to have 60 votes to pass. Only 43 of 52 Republicans voted for the amendment and no Democrats voted to move it forward.

Tacked onto that amendment was a proposal by Sen. Ted Cruz, R-Texas, that allows insurance companies to offer bare-bones, low-cost coverage plans. It also included a proposal by Sen. Rob Portman, R-Ohio, to provide billions of dollars of aid to low-income Americans who would have to move off of Medicaid onto private insurance plans.

On Wednesday the Senate is expected to consider another amendment to repeal Obamacare with an effective date two years away to give lawmakers time to come up with a new health care law.

"I applaud the Senate for taking a giant step to end the Obamacare nightmare," Trump said in a statement Tuesday. "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 Senate must now pass a bill and get it to my desk so we can finally end the Obamacare disaster once and for all."

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Tuesday's vote to start debate on health care is considered a win for McConnell even if it is just the first step on a steep path to repealing the health care law. Conservative and moderate senators had hinted in recent weeks that they may not support the “motion to proceed” because they were opposed to various versions of the bill the Senate was expected to consider.

"Look, we can't let this moment slip by," McConnell exhorted Republicans from the Senate floor just before the vote. "The people who sent us here expect us to begin this debate, to have the courage to tackle the tough issues."

McConnell said many Republicans didn't think they'd get the chance to replace Obamacare because they didn't think Trump would win the election last fall.

"But with a surprise election comes great opportunities to do things we never thought were possible," he said.

Democrats have urged Republicans to give up trying to repeal Obamacare and work with them to improve the law.

Senate Minority Leader Chuck Schumer, D-N.Y., pleaded in vain with his GOP colleagues to defeat the motion to proceed.

"At the moment, no one knows the plan that’s being cooked up in the Republican Leader’s office, but it seems to be his intention to do whatever it takes — to pass anything, no matter how small — to get the House and Senate Republicans into a conference on health care," Schumer said.

That process is bound to increase the deep Medicaid cuts that moderate GOP senators have opposed, he said.

"One last plea to my colleagues: Do not fall for the ruse that the Majority Leader is putting together," Schumer said. "We on this side are not fooled. Oh, no. And I hope my colleagues, who out of compassion and care for the people in their states have made such a fuss up to now, won't be fooled either."

Sen. Bob Corker, R-Tenn., said that everyone in the Senate chamber "understood" what would end up happening with the bill.

"We go through the amendment process ... at the end you end up in a situation where you vote on the lowest common denominator for passage," Corker said.

GOP leaders said there will be an open amendment process this week to allow Republicans and Democrats to help craft the health care bill.

"We've got our work cut out for us," said Sen. John Thune, R-S.D. "We cleared the first hurdle, but there is obviously a lot of work left to be done."

Sen. Chris Murphy, D-Conn., said he plans to offer 100 amendments as a delaying tactic to try to prevent Republicans from rushing the bill through this week. He said other Democratic senators also plan to offer dozens of amendments.

"We're going to need to create time on the floor in order for the people to see what the final bill is," Murphy said.

Contributing: Deirdre Shesgreen