Donald Trump has effectively proclaimed war on dozens of members of his own party, urging his supporters to “fight” the House Freedom Caucus in the upcoming midterm elections.

Members of the influential conservative group effectively helped kill the White House-approved health care plan last week, sparking several attacks from the President on Twitter.

But the bellicose leader signalled a dramatic intensification in his rift with the group, calling on his supporters to oppose Freedom Caucus members when they are up for re-election in 2018.

“The Freedom Caucus will hurt the entire Republican agenda if they don’t get on the team, & fast. We must fight them, & Dems, in 2018!” he wrote on Twitter.

The President initially blamed Democrats for the failure of his plan to repeal and replace Obamacare but has since refocused his attention toward the 32-member group of hard-line conservatives.

With a majority in the House of 239 to the Democrats’ 193, the Republican leader needs the support of the group to pass legislation.

“The Republican House Freedom Caucus was able to snatch defeat from the jaws of victory,” he tweeted on Monday. “After so many bad years they were ready for a win!”

House Speaker Paul Ryan, the top Republican in Congress who wrote the bill, said that he fears divisions in the Republican Party are pushing the President to work with Democrats.

“I don’t want that to happen. You know why? I want a patient-centered system, I don’t want government running health care,” he said in an interview with CBS this Morning. “The government shouldn’t tell you what you must do with your life, with your health care. We should give people choices.”

Speaking to reporters later in the day, Mr Ryan backed Mr Trump’s comments. “I understand the frustration,” he said. “I share the frustration.”

Mr Ryan repeated that “90 per cent” of his caucus is for the health care measure, but would not commit to a timeline on holding another vote.

Reacting to the President’s tweet, White House spokesperson Sarah Huckabee Sanders told reporters that there was “nothing to add at this time. The President’s tweet speaks for itself.”