The Senate on Tuesday began debate on healthcare legislation, taking a big step forward in its effort to repeal ObamaCare, after Vice President Pence broke a 50-50 tie to cast the deciding vote.

Two Republicans, Sens. Lisa Murkowski Lisa Ann MurkowskiGraham: GOP will confirm Trump's Supreme Court nominee before the election Gardner signals support for taking up Supreme Court nominee this year Tumultuous court battle upends fight for Senate MORE (Alaska) and Susan Collins Susan Margaret CollinsGraham: GOP will confirm Trump's Supreme Court nominee before the election Gardner signals support for taking up Supreme Court nominee this year Tumultuous court battle upends fight for Senate MORE (Maine), voted with Senate Democrats against the procedural measure, highlighting the narrow margin of error the White House and GOP leaders face in getting a repeal bill to President Trump.

Republicans would not have even gotten to 50 votes and Pence’s tie-breaker without Sen. John McCain John Sidney McCainBiden's six best bets in 2016 Trump states Replacing Justice Ginsburg could depend on Arizona's next senator The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE’s (R-Ariz.) dramatic return to the Senate for the first time since he was diagnosed with brain cancer.

McCain received bipartisan applause as he entered the chamber — and then cast a vote to keep the GOP hope of repealing ObamaCare alive.

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He said in a floor speech that he would vote against the Senate GOP’s current repeal-and-replace bill unless it is improved.

Just starting the debate was a victory for Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellGraham: GOP will confirm Trump's Supreme Court nominee before the election Trump puts Supreme Court fight at center of Ohio rally The Memo: Dems face balancing act on SCOTUS fight MORE (R-Ky.), who has had to postpone votes on multiple occasions because of opposition from conservative and centrist members of his conference, as well as McCain’s absence from Washington last week.

It was also a win for Trump, who has repeatedly pressured Republicans to begin their debate. Trump’s meeting at the White House with the Senate GOP conference was a turning point in the current effort, as it came at a time when GOP leaders seemed close to throwing in the towel.

McConnell praised the president’s contributions in a speech before the vote, even though Trump has at times chafed the GOP leader with some of his strategic moves and tweets.

That the GOP even had the 50 votes necessary to clear the first procedural hurdle only became clear when Sens. Dean Heller Dean Arthur HellerOn The Trail: Democrats plan to hammer Trump on Social Security, Medicare Lobbying World Democrats spend big to put Senate in play MORE (R-Nev.), Shelley Moore Capito Shelley Wellons Moore CapitoCongress must finish work on popular conservation bill before time runs out Second GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure MORE (R-W.Va.) and Rob Portman Robert (Rob) Jones PortmanMcConnell locks down key GOP votes in Supreme Court fight Romney undecided on authorizing subpoenas for GOP Obama-era probes Congress needs to prioritize government digital service delivery MORE (R-Ohio) announced minutes before the vote began that they would support their party.

All three faced enormous pressure to do so given calls from Trump, McConnell and other Republicans to let the debate move forward.

“Today, I will vote to begin debate to repeal and replace Obamacare,” Capito said in a statement. “As this process advances on the Senate floor, I will continue to make decisions that are in the best interest of West Virginians.”

While starting the debate is a victory as the GOP seeks to fulfill its years-long goal of repealing ObamaCare, what happens next is a bit of a mystery.

In a highly unusual situation, senators voted to begin debate without being sure of what plan they would end up voting on.

McConnell is expected to offer a measure repealing ObamaCare with a two-year delay, as well as a Senate bill that would repeal and replace the previous president’s signature legislation. Neither appears to have the votes to pass.

Some senators were predicting on Tuesday that a scaled-back “skinny” repeal bill might have the best chances of winning 50 votes, but even that is not clear.

That scaled-down measure would repeal ObamaCare’s individual and employer mandates, as well as the medical device tax, aides say.

If it passed, it would then allow Republicans to keep their process alive and go to a conference committee with the House to try to work out a new bill.

“I think the endgame is to be able to move something at the end of this process across the Senate floor that can get 50 votes and then get into conference with the House,” Sen. John Thune John Randolph ThuneGraham: GOP will confirm Trump's Supreme Court nominee before the election Tumultuous court battle upends fight for Senate What Senate Republicans have said about election-year Supreme Court vacancies MORE (R-S.D.) told reporters Tuesday afternoon.

Sen. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.) said he thought the final vote would be on the “lowest common denominator.”

McConnell had urged his party to back the procedural motion, arguing it was improper to prevent the Senate from holding a debate on such an important matter.

Just a week ago, it did not appear that his argument would win out.

Capito last week on Twitter said that she would only vote to advance to repeal legislation “if I am confident there is a replacement plan that addresses my concerns.”

Heller, who is facing a tough reelection race next year, voted to begin debate after having sharply criticized an earlier version of the ObamaCare replacement bill.

He argued, though, that he would not necessarily back the final bill.

“If the final product isn’t improved for the state of Nevada, then I will not vote for it; if it is improved, I will support it,” he said in a statement.

Whatever actions senators take will have dramatic results for the healthcare system.

The Congressional Budget Office (CBO) found that the latest version of the Senate’s replacement bill would cause 22 million more people to be uninsured over a decade. That bill would scale back ObamaCare subsidies to help people afford coverage and put a new cap on Medicaid spending, leading to billions of dollars in cuts to the program.

Even the “skinny” version would have dramatic effects and could destabilize the insurance market. The CBO previously found that repealing the individual mandate would lead to 15 million fewer people with insurance and increase premiums by about 20 percent.

But that bill could be changed in a conference committee with the House.

After McCain returned to cast his vote, he spoke from the Senate floor to urge both parties to work together. He said lawmakers should use the committee process, known as regular order, which was bypassed by GOP leaders on the healthcare bill.

“Why don’t we try the old way of legislating in the Senate, the way our rules and customs encourage us to act?” McCain said. “If this process ends in failure, which seem likely, then let’s return to regular order.”