(Bloomberg) -- Senate Republicans agreed to begin floor debateon health-care legislation, a hard-fought step amid uncertaintyabout exactly what plan senators will ultimately be asked to voteon.

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

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

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

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

“All we’ve managed to do was make more popular a policy thatwasn’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 canfinally end the Obamacare disaster once and for all,” he said in astatement.

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

"This is just the beginning,” McConnell told reporters. “We’renot 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’srepeal efforts.

“We will do everything we can inside this building,” SenateMinority Leader Chuck Schumer said at a news conference, addingthat people outside Washington should organize too. “We are goingto fight and fight and fight until this bill is dead.”

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

Markets Unmoved

Health-care stocks were unmoved by the vote, barely budgingafter senators approved the motion to begin debate. Hospitalstocks had dropped earlier in the day after HCA HealthcareInc. reported weaker-than-expected earnings results. The BI NorthAmerica Hospitals Valuation Peer Group was down 4.6 percent as of3:06 p.m. in New York.

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

The American Hospital Association said it was “disappointed” byTuesday’s vote, and that it opposes Medicaid cuts or otherlegislative action that would boost the number of people withouthealth insurance.

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

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

‘Clean Repeal’

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

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

McConnell teed up a version that includes a controversialamendment from Republican Ted Cruz of Texas that would allowinsurers to offer stripped-down plans that exclude people withpre-existing conditions, charge women more, and offer far skimpierbenefits. The version also includes language from Rob Portman ofOhio that would provide an additional $100 billion for thoseaffected by provisions rolling back Obamacare’s expansion ofMedicaid.

McConnell’s office said that the Congressional Budget Officehasn’t analyzed either the Cruz or Portman provisions, so theywould likely be subject to 60 votes, meaning Democrats could defeatthem.

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

But the parliamentarian said it doesn’t qualify under thefast-track procedures used by the Senate to pass the measure with50 votes.

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

QuickTake Q&A: This Unelected Official Holds Sway on HealthCare

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

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

White House Celebration

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

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

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

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