WASHINGTON _ Republicans narrowly advanced their campaign to roll back the Affordable Care Act on Tuesday, as the Senate voted by the slimmest of margins to begin debating legislation to repeal and potentially replace large sections of the 2010 law signed by President Barack Obama.

But the partisan 51-50 vote _ with Vice President Mike Pence breaking a tie — does not assure the success of the GOP's seven-year quest to dismantle the sweeping law, widely known as Obamacare.

With health coverage for tens of millions of Americans at stake, it remained unclear Tuesday what kind of health care bill — if any — might emerge by the time a final Senate vote is held, possibly as soon as Thursday.

In a moment of high drama, Tuesday's vote was saved by the return of Sen. John McCain, R-Ariz., following his recent surgery and his diagnosis last week of brain cancer. He arrived in the chamber to cheers and a standing ovation from his colleagues before casting a crucial vote to back Senate Majority Leader Mitch McConnell, R-Ky.

But McCain then delivered a speech criticizing the process the Republican leader had used to push the repeal legislation forward, and called for more bipartisanship.

"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 seems likely, then let's return to regular order."

McConnell has set up a series of votes this week on competing proposals to repeal much of Obamacare, or to repeal and replace pieces of the law. The first one, to consider what had been seen as the GOP's leading replacement proposal, failed Tuesday evening by a vote of 43-57, with nine Republicans voting against it.

Nevertheless, Tuesday's procedural vote to kick off that process marked a victory for McConnell, who has been laboring for months behind closed doors to rally his divided caucus, and for President Donald Trump, who has been publicly calling on GOP senators to stick with the repeal push.

Trump praised the Senate vote to move to debate as a "big step" during a news conference at the White House. "We're going to give you great health care," the president said, adding that Republicans would, "over the next week or two, come up with a plan."

To get to this point, McConnell had to abandon the customary legislative process, foregoing public hearings and committee debate in a way almost never seen for major legislative proposals.

And he left Republican lawmakers with a series of legislative options _ which polls show are deeply unpopular with Americans _ that would leave as many as 32 million more people without health coverage and weaken health protections for tens of millions more.

The GOP plans have been widely panned by independent analysts and vigorously opposed by every major patient advocacy group and every leading organization representing physicians, nurses or hospitals.

Opponents include the American Diabetes Association, the March of Dimes, AARP, the American Medical Association, the American Lung Association, the National Multiple Sclerosis Society, the American Heart Association and the advocacy arm of the American Cancer Society.

Before the vote, Senate Minority Leader Charles E. Schumer, D-N.Y., pleaded with GOP lawmakers in the Senate chamber to stop their rush to repeal and instead work with Democrats on bipartisan fixes to the current law.

As voting began, protesters filled the gallery, chanting "Kill the bill!" and "Shame!" before being escorted out of the chamber.

The proceedings came to a momentary standstill as Sen. Ron Johnson, R-Wis., who had yet to vote, huddled with McConnell at the majority leader's desk. But Sens. Susan Collins of Maine and Lisa Murkowski of Alaska were the only Republicans who voted against the motion.

Democratic senators all voted against the measure after the Republicans finished voting.

Patient advocates and health care groups condemned the Senate vote.

"It's outrageous and irresponsible that the Senate voted to proceed to final passage on such a devastating and destructive bill, without any committee consideration, expert testimony, hearings or even a public draft of what they will vote on," said Anthony Wright, executive director of Health Access California.

As Republicans now move to crafting a final bill, one leading GOP option, favored by conservatives, is to simply repeal most major parts of Obamacare and then try to pass a new health care law in the future.

McConnell is also expected to offer a revised version of the Republican bill, the Better Care Reconciliation Act, which would replace most of the law while at the same time dramatically scaling back federal funding of the Medicaid insurance program that covers about 74 million low-income, elderly and disabled Americans.

So far, neither of these approaches has garnered the necessary 50 votes from the Republicans' 52-seat Senate majority, and that math does not appear to have changed despite Tuesday's vote to begin debate.

Tuesday evening, leaders offered their first amendment, which was the BCRA with two provisions added _ one from Sen. Ted Cruz, R-Texas, to allow health insurers to sell skimpier plans that do not include basic benefits such as prescription drugs, maternity care and mental health and substance abuse services, and another from Sen. Rob Portman, R-Ohio, that would add $100 billion to help those losing Medicaid.

That package failed to reach the 60-vote threshold needed for passage under budget rules.

The other proposals would need only 51 votes using special budget rules.

GOP centrists have complained that all of those plans would leave too many Americans without insurance, and have urged McConnell to start the process over, working on a bipartisan approach that would protect those covered under Obamacare.

The lack of a GOP consensus prompted Senate Republican leaders to float a new idea Tuesday _ a far more limited repeal bill that would target the most unpopular parts of Obamacare, including its insurance mandates.

That plan _ dubbed "skinny repeal" _ would eliminate only a medical device tax established by Obamacare and two mandates requiring Americans to have coverage and requiring large employers to offer health benefits.

The approach would fall far short of what Republicans have been promising for years. But if it passes, Senate Republicans could at least send something to a conference committee where lawmakers could work out a compromise with a sweeping repeal bill passed by House Republicans in May, giving them yet another shot at unwinding Obamacare.

The skinny repeal plan risks major disruptions to the nation's insurance markets, as the lack of any insurance mandate could free Americans to buy health insurance only when they get sick, driving up costs.

But it would leave in place other crucial parts of the 2010 law, including hundreds of billions of dollars of federal aid to state Medicaid programs and insurance subsidies that help low- and moderate-income Americans.

That assistance is credited with helping extend coverage to more than 20 million previously uninsured Americans.

Also untouched would be landmark rules established by Obamacare that require insurers to cover a basic set of benefits and prohibit health plans from discriminating against Americans with pre-existing medical conditions.

It remains unclear whether House Republicans would agree to this more limited repeal plan. But with centrist and conservative GOP senators in deep disagreement, the skinny repeal may be all McConnell can get through.

The repeal-only plan would leave an estimated 32 million more people without health coverage over the next decade, according to the nonpartisan Congressional Budget Office, or CBO.

The alternative proposed by McConnell that repeals and replaces major parts of the law while also dramatically scaling back federal Medicaid assistance is only slightly less disruptive, CBO analysts have concluded, and would leave 22 million more Americans without insurance by 2026.