US President Donald Trump has warned Republican lawmakers that voters could punish them if they do not approve a plan he favours to dismantle Obamacare, as pressure grows on him to win the first major legislative battle of his presidency.

In one of the few visits he has made to the US Capitol since taking office, Trump told fellow Republicans in the House of Representatives they would face "political problems" for opposing the bill that takes apart Obamacare and partially replaces it.

His comments were interpreted by lawmakers speaking to reporters as a threat that they would lose their seats in the next elections.

Some conservative lawmakers believe the healthcare bill does not go far enough, while moderate Republicans worry that millions of Americans will be hurt by the dismantling of the 2010 Affordable Care Act, Democratic former president Barack Obama's signature healthcare legislation.

"If we fail to get it done, fail to [meet] the promises made by all of us, including the president, then it could have a very detrimental effect to Republicans in '18 who are running for re-election," said Republican Congressman Mike Conaway. "If it fails, then there will be a lot of people looking for work in 2018."

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Party leaders hope to move the bill to the House of Representatives floor for debate as early as Thursday. But the administration and House leadership can afford to lose only about 20 votes from Republican ranks or risk the bill failing since Democrats are united against it.

Republican Congressman Mark Meadows, chairman of the conservative House Freedom Caucus, said the caucus has decided not to vote as a bloc on the bill, meaning Republican leadership could still win votes from some of the group's roughly three dozen members.

In a tweet, NBC news channel listed 26 House Republicans who have said they cannot support the measure.

NBC News has now ID'ed 26 House GOPers who are opposed/leaning strongly against House health-care bill, per @AlexNBCNews & @LACaldwellDC pic.twitter.com/lksvwOTw3x — Mark Murray (@mmurraypolitics) March 21, 2017

Repealing and replacing Obamacare was one of Trump's main campaign promises and has been a goal of Republicans since it was enacted.

While Trump predicted that Republicans could face challenges in primary contests ahead of the 2018 midterm elections if they do not gut Obamacare, there is also danger to them in doing so. If the Republican bill is passed, millions of voters might lose their healthcare coverage.

The Congressional Budget Office said last week that 14 million people would forfeit coverage under the House bill over the next year, although that number could change based on the most current version of the legislation.

Republican leaders tweaked the bill this week to try to satisfy critics, mainly from their own party.

Republican chairmen for two key committees said late on Monday they proposed more funding for tax credits, which conservatives have opposed, that would give the Senate flexibility to help older people afford health insurance. Additionally, Obamacare's taxes would be eliminated in 2017 instead of 2018.

The amendments also addressed Medicaid, which is the country's largest health insurance programme and covers about 70 million people, mostly the poor. The changes would allow states to implement work requirements for certain adults, an idea championed by many conservatives, and to decide how they receive federal funds.