Donald Trump has effectively declared 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.

“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!"

Responding to the President’s Thursday 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."

With a majority in the House of 239 to the Democrats' 193, the Republican leader needs the support of the group to smoothly pass legislation. The unified opposition by Democrats, means Mr Trump cannot afford to lose many Republicans as he tries to get his agenda through Congress, including healthcare, tax cuts and infrastructure spending. But keeping Freedom Caucus members happy without losing the votes of Republican moderates has proven tough.

Representative Justin Amash, a Freedom Caucus member from Michigan, shot back immediately at Mr Trump in remarks outside the US Capitol.

“Most people don't take well to being bullied,” Mr Amash said. Asked if Mr Trump's comments were constructive, Mr Amash added: “It's constructive in [school]. It may allow a child to get his way, but that's not how our government works.”

Mr Amash also depicted Mr Trump as beholden to the Washington establishment. “It didn't take long for the swamp to drain @realDonaldTrump,” Mr Amash wrote on Twitter. “No shame, Mr. President. Almost everyone succumbs to the D.C. Establishment.”

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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.

Those comments were criticised by Republican Senator Bob Corker, who said: “It’s not the kind of thing that the leader, the speaker of a House, should be saying.”

Most of the GOP no-voters represent safely Republican seats — some drawn to ensure they stay that way — and whose grip on power would only be threatened by a primary challenger. They have also taken a stand knowing that while Mr Trump is popular in their districts, they are equally so.

But some Republicans seemed emboldened by the White House's prodding: Grand Rapids businessman Brian Ellis challenged Mr Amash to a primary in 2014 and said he may do so him again.

“It's the same old thing with the Freedom Caucus: Don't let the perfect be the enemy of the good,” Mr Ellis said. “The Freedom Caucus didn't care. And for Amash to reject them is to not reflect what I know to be the desire of the constituents of this district.”