US President Donald Trump may struggle to pass his Obamacare replacement bill after the House Freedom Caucus said it would oppose the legislation in a dramatic turn of events.

Despite the Republicans' best efforts to persuade the caucus to vote in favour of the bill, at least 25 members of the conservative group remain unconvinced by Trump's proposed replacement.

"The opposition is still strong," Chairman of the group Mark Meadows told The Hill.

"They don't have the votes to pass this tomorrow. We believe that they need to start over and do a bill that actually reduces premiums."

There is room for around 22 defections if the GOP is to get the bill through the House, but with at least 25 members of the caucus unconvinced, the bill as it stands appears unlikely to pass.

Trump strongly suggested to members of the caucus on Tuesday (21 March) that their political futures would be uncertain if they stood in the way of his replacement for the Affordable Care Act, dubbed 'Trumpcare'.

"I honestly think many of you will lose your seats in 2018 if you don't get this done," the President told members of the Freedom Caucus, with Representative John Duncan telling The Hill after the meeting: "He told us if we don't pass this bill on Thursday, it will put everything in jeopardy that he wants to do, his agenda."

However, despite Trump's warning it appears members of the caucus are not prepared to budge – putting the future of the bill in jeopardy. Members of the caucus believe insurance premiums should be lower – and that the bill should scrap essential healthcare benefits laid out by Obamacare that they believe keep premiums high.

And members of the caucus attended a meeting with VP Mike Pence at the White House on Wednesday (22 March), but it appears little leeway was made.

Indeed, it has been suggested the vote on the bill should be postponed in order to avoid a showdown and allow final details to be hammered out futher, but GOP leaders appeared to be forging ahead with a Thursday vote.