WASHINGTON -- U.S. President Donald Trump accused Alaska Sen. Lisa Murkowski, a fellow Republican, of disappointing the country by opposing the GOP effort to demolish the Obama health care law, after initial votes demonstrated the party will be hard pressed to make any sweeping changes in the statute.

Senators planned to vote Wednesday on a Republican amendment to repeal much of President Barack Obama's law and give Congress two years to come up with a replacement. That was expected to be rejected by a combination of solidly opposed Democrats and Republicans unwilling to tear down the law without a replacement in hand.

"Now we have to keep working hard," Senate Majority Leader Mitch McConnell, R-Ky., said Wednesday. "We're determined to do everything we can to succeed. We know our constituents are counting on us."

In an initial GOP setback, the Senate voted 57-43 Tuesday to block a wide-ranging amendment by Majority Leader Mitch McConnell of Kentucky to replace Obama's statute with a more restrictive substitute. Those voting no included nine Republicans, ranging from conservative Mike Lee of Utah to Alaska moderate Murkowski, in a roll call that raised questions about what if any reshaping of Obama's law splintered Republicans can muster votes to achieve.

Trump took to Twitter early Wednesday to single out Murkowski.

"Senator Γåòlisamurkowski of the Great State of Alaska really let the Republicans, and our country, down yesterday. Too bad!" he wrote.

Murkowski, a senator since 2002, was re-elected last fall and has criticized the GOP's proposed cuts in the Medicaid health insurance program for the poor, the disabled and nursing home patients. During the presidential campaign last October, she said she would not support Trump after tapes were released of him making crude comments about women in 2005.

Asked about Trump's comment, Murkowski said, "I don't really follow Twitter that much."

Tuesday's rejected GOP amendment was centred on language by McConnell erasing Obama's tax penalties on people who don't buy insurance, cutting Medicaid and trimming subsidies for consumers. It included a provision by Ted Cruz, R-Texas, letting insurers sell cut-rate policies with skimpy coverage plus an additional $100 billion -- sought by Midwestern moderates including Rob Portman, R-Ohio -- to help states ease out-of-pocket costs for people losing Medicaid.

GOP defectors also included Sens. Dean Heller of Nevada, who faces a tough re-election fight next year, and usually steady McConnell allies Bob Corker of Tennessee, Lindsey Graham of South Carolina and Jerry Moran of Kansas.

Before that defeat, Trump and McConnell had gained a reprieve from what seemed a likely defeat and won a 51-50 vote to begin debating the GOP health care measure, which sits atop the party's legislative priorities.

In a day of thrilling political theatre, Vice-President Mike Pence broke a tie roll call after Sen. John McCain returned to the Capitol from his struggle against brain cancer to help push the bill over the top. There were defections from just two of the 52 GOP senators -- Murkowski and Maine's Susan Collins -- the precise number McConnell could afford to lose and still carry the day.

All Democrats voted against even starting debate on legislation to dismantle the 2010 statute that stands as President Obama's landmark domestic achievement.

Leaders were openly discussing a "skinny bill" repealing unpopular parts of the statute like its tax penalties on people not buying coverage -- a tactic aimed chiefly at letting Senate-House bargainers seek a final compromise later.

McConnell's Democratic counterpart, Senate Minority Leader Chuck Schumer of New York, saw the GOP's path as bleak.

"It seems the Republican majority is no clearer on what the end game is, because there's no good way out of this," he said.

Senators were working their way through 20 hours of debate. At week's end, a "vote-a-rama" of rapid-fire voting on a mountain of amendments was expected before moving to final passage -- of something.

Internal GOP differences remain over how broadly to repeal the law, how to reimburse states that would suffer from the bill's Medicaid cuts and whether to let insurers sell cut-rate, bare-bones coverage that falls short of the requirements.

While pressure and deal-making helped win over vacillating Republicans to begin debate, they remained fragmented over what to do next. Several pointedly left open the possibility of opposing the final bill if it didn't suit their states.

Even McCain, R-Ariz., who received a warm standing ovation and bipartisan hugs when he returned, said he'd oppose the final bill if it didn't reflect changes to help his state and lambasted the roughshod process his own party was using.

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Associated Press writers Erica Werner, Stephen Ohlemacher, Jill Colvin, Ricardo Alonso-Zaldivar and Mary Clare Jalonick contributed to this report.