President Donald Trump barged into Senate Republicans' delicate health care negotiations, declaring that if lawmakers can't reach a deal they should simply repeal "Obamacare" right away and then replace it later on.

Trump's tweet on Friday revives an approach that GOP leaders and the president himself considered but dismissed months ago as impractical and politically unwise. And it's likely to further complicate Majority Leader Mitch McConnell's task as he struggles to bridge the divide between GOP moderates and conservatives as senators leave Washington for the Fourth of July break without having voted on a health care bill as planned.

"If Republican Senators are unable to pass what they are working on now, they should immediately REPEAL, and then REPLACE at a later date!" Trump wrote.

The president sent his early-morning tweet shortly after Nebraska Republican Senator Ben Sasse appeared on Fox News Channel's "Fox & Friends" to talk about a letter he had sent to Trump making that exact suggestion: a vote on repealing former President Barack Obama's health law followed by a new effort at a working out a replacement.

Trump is a known "Fox & Friends" viewer, but Republican Senator Rand Paul of Kentucky also claimed credit for recommending the tactic to the president in a conversation earlier in the week.

"Senator Rand Paul suggested this very idea to the president," said Paul spokesman Sergio Gor. "The senator fully agrees that we must immediately repeal Obamacare and then work on replacing it right away." 

Either way, Trump's suggestion has the potential to harden divisions within the GOP as conservatives like Paul and Sasse complain that McConnell's bill does not go far enough in repealing Obama's health care law while moderates criticise it as overly harsh in kicking people off insurance roles, shrinking the Medicaid safety net and increasing premiums for older Americans.

McConnell told reporters after an event on Friday in his home state of Kentucky that the health care bill remains challenging but "we are going to stick with that path." 

"It's not easy making American great again, is it?" McConnell said.

McConnell, Representative from the state of Kentucky, has been trying to strike deals with members of both factions in order to finalise a rewritten bill lawmakers can vote on when they return to the Capitol the second week of July.

Even before Trump weighed in, though, it wasn't clear how far he was getting, and Trump's tweet did not appear to suggest a lot of White House confidence in the outcome.

"McConnell's trying to achieve a 50-vote Venn diagram between some very competing factions," said Rodney Whitlock, a veteran health policy expert who worked as a Senate GOP aide during passage of the Democrats' Affordable Care Act.

"So what the president tweeted takes one side of that Venn diagram and pushes it further away, and actually puts on the table an option that will probably drive that group away from seeking compromise with the other side of the Venn diagram." 

Even before Trump was inaugurated in January, Republicans had debated and ultimately discarded the idea of repealing Obamacare before replacing it, concluding that both must happen simultaneously.