WASHINGTON (AP) — President Donald Trump urged Republicans to "step up to the plate" for Tuesday's crucial Senate vote on their bill eviscerating much of the Obama health care law. A cliff-hanger roll call was likely, with added drama from Sen. John McCain's return to the Capitol for his first vote since being diagnosed with brain cancer.

No stranger to heroic episodes, the Navy pilot who persevered through five years of captivity during the Vietnam War announced he'd be back in Washington for the critical roll call on beginning debate on the legislation. The 80-year-old had been at home in Arizona since he revealed last week that he's undergoing treatment for the disease, but a statement said he "looks forward" to returning for work on health care and other legislation.

As the initial vote approached, it remained unclear exactly which version of the legislation Senate Majority Leader Mitch McConnell, R-Ky., would put in play. Republicans expect McCain to support taking up the measure, and his mere presence could make it harder for wavering Republicans to cast a vote against even considering the bill.

Keeping pressure on GOP senators, McConnell noted that many "made commitments to our constituents to provide relief from this failed left-wing experiment. And now we have a real opportunity to keep those commitments." He added, "I hope everyone will seize the moment."

In one sign of progress, conservative Sen. Rand Paul, R-Ky., said he would support commencing debate. He said McConnell told him the Senate would debate Paul's proposal to scuttle much of Obama's law and give Congress two years to enact a replacement — an amendment that seemed certain to lose.

Democrats uniformly oppose the effort to tear down President Barack Obama's signature legislative achievement. Republicans control the chamber 52-48, meaning they can afford to lose just two Republicans with McCain around and only one in his absence. Vice President Mike Pence would cast a tie-breaking vote.

Trump kept up the pressure on GOP lawmakers, tweeting that "After 7 years of talking, we will soon see whether or not Republicans are willing to step up to the plate!" He added: "ObamaCare is torturing the American People. The Democrats have fooled the people long enough. Repeal or Repeal & Replace! I have pen in hand."

McConnell's bill would abolish much of Obama's law, eliminating its tax penalties on people not buying policies, cutting Medicaid, eliminating its tax boosts on medical companies and providing less generous health care subsidies for consumers. But at least a dozen GOP senators have openly said they oppose or criticized the measure, which McConnell has revised as he's hunted Republican support.

While the first vote had long seemed headed toward defeat, Republicans began showing glimmers of optimism. Senators and aides said talks were continuing aimed at winning over enough Republicans to commence debate.

Besides allowing an early vote on Paul's repeal plan, moderates were seeking additional money for states that would be hurt by cuts in Medicaid, the health insurance program for the poor, the disabled and nursing home patients. Conservatives wanted a vote on a proposal by Ted Cruz, R-Texas, letting insurers offer bare-bones policies with low premiums, which would be illegal under Obama's law.

With leaders still struggling to line up enough votes to approve a wide-ranging overhaul of Obama's law, there was talk of eventually trying to pass a narrow bill — details still unclear — so House-Senate bargainers could craft a compromise. That, too, was encountering problems.

"This idea that we're going to vote on something just to get in conference and then figure it out later is nuts," Sen. Lindsey Graham, R-S.C., told reporters.

Should Tuesday's vote fail, it would be an unalloyed embarrassment for a party that finally gained control of the White House, Senate and House in January but still fell flat on its promise to uproot Obamacare. Republicans could try returning to the bill later this year if they somehow round up more support.

Should the initial motion win, that would prompt 20 hours of debate and countless amendments in a battle likely to last all week. And even then, the measure's ultimate fate still seemed iffy because of GOP divisions.

Obama's law was enacted in 2010 over unanimous Republican opposition. Since then, its expansion of Medicaid and creation of federal insurance marketplaces has produced 20 million fewer uninsured people. It's also provided protections that require insurers to provide robust coverage to all, cap consumers' annual and lifetime expenditures and ensure that people with serious medical conditions pay the same premiums as the healthy.

The law has been unpopular with GOP voters and the party has launched numerous attempts to dismantle the statute. All until this year were mere aspirations because Obama vetoed every major one that reached him.

Ever since 2010, Republicans have been largely united on scuttling the statute but divided over how to replace it.

Those divides sharpened with Trump willing to sign legislation and estimates by the nonpartisan Congressional Budget Office that several GOP bills would cause more than 20 million people to become uninsured by 2026. Polls showing growing popularity for Obama's law and abysmal approval ratings for the GOP effort haven't helped.

The House approved its health care bill in May after several setbacks. With the House planning to begin summer recess this weekend, Majority Leader Kevin McCarthy, R-Calif., said the chamber would return to "finish the job" by considering whatever the Senate might approve.

Moderate Sen. Susan Collins, R-Maine, has remained opposed to beginning debate on any option McConnell has revealed so far.

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Associated Press writers Erica Werner and Andrew Taylor contributed to this report.