It is being described as a setback for President Trump, putting a question mark over his promise to quickly replace Obamacare.
A vote on his new healthcare plan has been postponed in the House of Representatives, with not enough Republicans giving their support.
There is talk of a new vote on Friday, but some reports say that is not certain.
Republican representative Mark Meadows, the Chairman of the US House Freedom Caucus, told reporters: “We have not gotten enough of our members to get to ‘yes’ at this point, under what we are currently considering.
“However, I would say progress is being made.”
Democrat Nancy Pelosi, the House Minority Leader, said: “As bad as Trumpcare was, Republicans have been up to all night trying to make it worse. They’re scrambling to find a bill that they can pass on the floor.
“I don’t know if you want to call this on Trump’s part a ‘rookie’s error’. But you don’t find a day and say we’re going to pass a bill. you build your consensus in your caucus. And when you’re ready, you set the date to bring it to the floor.”
The Republicans can only afford to have 21 of their own representatives vote against the bill; some US media report that at the moment there are 26 Republicans opposed.
The lack of support comes despite Trump going to Congress himself to try to get Republicans on side.