(Bloomberg) -- Senate Republicans agreed to begin floor debate on health-care legislation, a hard-fought step amid uncertainty about exactly what plan senators will ultimately be asked to vote on.

The drama of Tuesday’s 51-50 vote -- with Vice President Mike Pence providing the tie-breaker -- was heightened by the arrival from Arizona of Senator John McCain to help the GOP try to repeal Obamacare following his brain-cancer diagnosis last week.

Majority Leader Mitch McConnell of Kentucky began with the debate with two amendments -- one that mirrors an Obamacare repeal bill passed in 2015 by the Senate and vetoed by President Barack Obama and another that contains the most recent version of McConnell’s replacement health bill. Both are likely to fail.

McCain entered the chamber to applause from both Republicans and Democrats, but then fired off a warning to his own leadership, saying he won’t vote to pass the latest version of the GOP health bill.

“We keep trying to win without help from the other side of the aisle,” McCain said in a speech on the Senate floor after the vote. “We are getting nothing done, my friends, we’re getting nothing done.”

“All we’ve managed to do was make more popular a policy that wasn’t very popular,” he added, referring to Obamacare.

‘Giant Step’

President Donald Trump praised Senate Republicans for the vote, calling it a “giant step” to begin dismantling Obamacare.

“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,” he said in a statement.

McConnell is aiming for a final vote this week, but the path is unclear and it’s far from certain he’ll get the votes to pass a final bill. John Thune of South Dakota, the No. 3 Senate Republican, said he expects the Senate to hold an all-night session Thursday into Friday, known as a "vote-a-rama," where senators in both parties will be able to offer nearly unlimited numbers of amendments.

"This is just the beginning,” McConnell told reporters. “We’re not out here to spike the football.”

As the vote began, chants of “Kill the bill, don’t kill us!” and “Shame!” broke out in the Senate visitors’ gallery.

Senate Democrats pledged to fight against all of the GOP’s repeal efforts.

“We will do everything we can inside this building,” Senate Minority Leader Chuck Schumer said at a news conference, adding that people outside Washington should organize too. “We are going to fight and fight and fight until this bill is dead.”

Republican leaders have promised senators they’ll each get a chance to vote on their preferred plan, with a final measure to be put together by leadership at the end of the debate. Among the proposals is a simple repeal of Obamacare with a two-year delay, which McConnell has said Trump would sign. Senators may also consider a more stripped-down repeal bill that eliminates the individual mandate along with a few key elements of Obamacare.

Markets Unmoved

Health-care stocks were unmoved by the vote, barely budging after senators approved the motion to begin debate. Hospital stocks had dropped earlier in the day after HCA Healthcare Inc. reported weaker-than-expected earnings results. The BI North America Hospitals Valuation Peer Group was down 4.6 percent as of 3:06 p.m. in New York. 

Health insurers, many of which have already pulled back from Obamacare’s markets, were little changed, and the Standard and Poor’s 500 Managed Health Care Index was down less than 1 percent.

The American Hospital Association said it was “disappointed” by Tuesday’s vote, and that it opposes Medicaid cuts or other legislative action that would boost the number of people without health insurance. 

“Although we are certainly disappointed by today’s development, we are also more determined than ever to help advance solutions aimed at protecting coverage,” Rick Pollack, the hospital group’s chief executive officer, said in a statement. “Any legislative efforts that entail devastating cuts to the Medicaid program or coverage losses will be opposed.”

The House passed its Obamacare replacement plan in May, and Trump urged the GOP on Twitter Tuesday to "step up to the plate!"

‘Clean Repeal’

Senator Rand Paul of Kentucky, who had insisted on a simple-repeal vote, said on Twitter that if it fails, the GOP will vote on "whatever version of CLEAN repeal we can pass."

Another possibility is a repeal-and-replace version that McConnell has been revising. 

McConnell teed up a version that includes a controversial amendment from Republican Ted Cruz of Texas that would allow insurers to offer stripped-down plans that exclude people with pre-existing conditions, charge women more, and offer far skimpier benefits. The version also includes language from Rob Portman of Ohio that would provide an additional $100 billion for those affected by provisions rolling back Obamacare’s expansion of Medicaid.

McConnell’s office said that the Congressional Budget Office hasn’t analyzed either the Cruz or Portman provisions, so they would likely be subject to 60 votes, meaning Democrats could defeat them.

In addition, the Senate parliamentarian issued more rulings that complicate GOP efforts to change Obamacare’s rules on how insurance prices are set. The Senate GOP proposal would have allowed older people to be charged as much as five times more than young people, compared with the 3-to-1 ratio under Obamacare. The change would have resulted in lower premiums for young people, and was backed by by Republican lawmakers who hoped it would draw more young people into the market. 

But the parliamentarian said it doesn’t qualify under the fast-track procedures used by the Senate to pass the measure with 50 votes.

The Senate is also likely to consider a so-called skinny repeal that would repeal part of Obamacare -- the requirements that most individuals obtain insurance and that most employers offer it to their workers, and perhaps the tax on medical devices.

QuickTake Q&A: This Unelected Official Holds Sway on Health Care

"Everything’s on the table. It’s whatever we can get out of the Senate and get to a conference" with the House to reach a compromise, said second-ranking Senate Republican John Cornyn of Texas.

The House bill, H.R. 1628, would wind down an expansion of Medicaid insurance for the poor and eliminate $1 trillion in taxes on the wealthy, insurers and drugmakers used to fund the law. Republicans say it would allow a market-based system that would let people make more health-coverage decisions for themselves. It would replace Obamacare subsidies with tax credits based primarily on age that phase out for people with incomes above $75,000.

White House Celebration

Trump and House Republicans celebrated the May 4 passage of the bill with an outdoor ceremony at the White House, though the president later called the bill "mean." The nonpartisan CBO said it would cause millions to lose health coverage.

Trump has vacillated on what to do about health care, at times saying Congress should let Obamacare die and then come up with a replacement, and at other times insisting that coming up with a replacement now would be a better plan.

After the House vote, McConnell crafted a Senate repeal-and-replace plan in secret, but last week he was forced to admit it lacked enough GOP support. The CBO said it would cause 22 million fewer people to have health care by 2016, similar to estimates for the House bill. The majority leader also acknowledged that a repeal-only bill lacked the votes to advance.

Senator Susan Collins of Maine has said she opposed repealing Obamacare without an adequate replacement and expressed concern about cuts in Medicaid coverage for low-income Americans.