WASHINGTON • United States Senate Republican leaders postponed a vote on a healthcare overhaul on Tuesday after resistance from members of their own party.

President Donald Trump then summoned Republican senators to the White House to urge them to break the impasse.

The delay put the future of a long-time top Republican priority in doubt amid concerns about the Senate Bill from both moderate and conservative Republicans.

With Democrats united in their opposition, Republicans can afford to lose only two votes among their own ranks in the Senate.

Senate Majority Leader Mitch McConnell had been pushing for a vote ahead of the recess that starts at the end of the week. The legislation would repeal major elements of Obamacare and shrink the Medicaid government healthcare programme for the poor.

"We're going to press on," Mr McConnell said after announcing the delay, adding that leaders would keep working to make senators "comfortable" with the Bill. "We're optimistic we're going to get to a result that is better than the status quo."

At the White House meeting with most of the 52 Republican senators, Mr Trump said it was vital to reach an agreement on the Senate healthcare measure because Obamacare was "melting down".

"So we're going to talk and we're going to see what we can do. We're getting very close," Mr Trump told the senators. But he added: "If we don't get it done, it's just going to be something that we're not going to like, and that's okay."

Mr McConnell, whose party has a razor-thin majority in the 100-member Senate, told reporters that Republican leaders would work through the week to win over the 50 senators needed to pass the Bill, with a vote planned after the recess.

Vice-President Mike Pence could provide the crucial vote needed to break a tie.

The House of Representatives last month passed its own version of a healthcare Bill, but the Senate Bill has been criticised from both the left and the right. Moderate Republicans are worried millions of people would lose their insurance. Conservatives said the Bill does not do enough to erase Obamacare.

The Bill's prospects were not helped by a Congressional Budget Office (CBO) analysis on Monday saying it would cause 22 million Americans to lose insurance over the next decade, although it would reduce the federal deficit by US$321 billion (S$445 billion) over that period. The report prompted Senator Susan Collins, a Republican moderate, to say she could not support the Bill as it stands.

At least four conservative Republican senators said they were still opposed after the CBO analysis.

US stock prices fell, as the decision to postpone the vote added to investor worries about Mr Trump's ability to deliver on his promises of tax reform and deregulation, as well as changes to the health sector. Those expected changes have driven a rally in US stocks this year.

The benchmark S&P 500 index closed down 0.8 per cent, and the Dow Jones industrial average finished 0.46 per cent lower.

REUTERS