Washington - US Republicans clashed on Wednesday over plans to replace Obamacare, with President Donald Trump and top allies in Congress pitching a health care plan that is staring at defeat unless party rebels' demands for changes are met.Seven years after Barack Obama's landmark reform became law - a period when Republicans repeatedly tried and failed to yank it out by its roots - the House and Senate will soon vote on a "repeal and replace" effort that hangs in the balance.After facing sharp resistance from rank-and-file Republicans, House Speaker Paul Ryan acknowledged late on Wednesday that changes would have to be made to get the legislation over the finish line.Ryan said he envisioned "necessary improvements and refinements" made to the bill.Widening concernsA crucial test comes on Thursday, when the House Budget Committee votes on the plan. Republican Dave Brat said he will oppose it.He is one of three members of the far-right Freedom Caucus on the 36-member Budget panel. If the three vote no, along with another Republican and united Democratic opposition, the measure would fail to advance.Staunch conservatives like Brat hate the new plan's similarities to Obama's law. Moderate Republicans are nervous that the bill winding through Congress would cause many struggling families to suffer, a prospect highlighted by a damning congressional projection that 24 million people could lose insurance within a decade under the plan.And Senator Rand Paul, a key opponent who wants the entire plan scrapped in favour of a clean repeal bill, went so far as to accuse Ryan of misleading Trump about what is in the legislation.Ryan "is selling him a bill of goods that he didn't explain to the president," Paul told reporters at a Washington rally where he urged conservatives to "bring down the Paul Ryan plan."Trump and congressional Republicans campaigned relentlessly last year on a pledge to repeal the Affordable Care Act - popularly known as Obamacare - and replace it with something that lowers premium costs and allows more choice for consumers.But the solution introduced in the House, already facing united opposition from Democrats, has been eviscerated by some in Trump's own party.Enough opposition has swelled from Republicans in the Senate, where the party holds a narrow 52-48 majority, that the bill almost certainly will not pass without major changes."All of us are concerned about it," Republican Senator John McCain told AFP.He is one of several senators raising warning flags over the plan's ending the expansion of Medicaid, the health care program for the poor and disabled.The Congressional Budget Office on Monday projected that millions of Americans could lose their coverage if federal funding dries up for the expansion."My state is a Medicaid state, so obviously we have to address that," McCain said.'Catastrophic Situation'Trump took his sales pitch on the road on Wednesday, holding a rowdy campaign-style rally in Nashville, Tennessee where he warned that Americans were facing a "catastrophic situation" with millions of people losing coverage should Obamcare not be repealed immediately."We want Americans to be able to purchase the health insurance they want, not the plans forced on them by our government," he said.But he conceded that "negotiation" - even with fellow Republicans - was the path forward."We will arbitrate, we will all get together, we will get something done," Trump said.Ryan, acknowledging "refinements and improvements" to the bill were likely, meanwhile appeared to reject the prospect of an overhaul."The major components are staying intact, because this is something we wrote with President Trump," he told Fox News.On Wednesday, Vice President Mike Pence headed to Capitol Hill to rally Republicans to the cause in a closed-door gathering.Freedom Caucus chairperson Mark Meadows said the bill needed to be reworked."Ultimately it's going to have to be amended in order to gain enough support to get passage on the House floor," he said.The Republican plan replaces Obamacare's government subsidies with tax credits to help people buy insurance, but critics say the credits are not enough to cover insurance costs for lower-income families.Meadows said one of his priorities is to add a work requirement for Medicaid recipients.Several Republicans, including some governors in states that expanded Medicaid, believe such expansion to low-income adults without disabilities gives them an incentive not to work.Senate Republican John Barrasso downplayed the divisions."There's still plenty of time to work on this," he said. "We need to get it right because the status quo cannot hold."