President Trump on Thursday struck back at the House caucus that sunk his ObamaCare replacement bill, threatening their legislative careers if the staunchly conservative members refuse to get on board with the new president’s agenda.

“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!” Trump tweeted.

Later Thursday, Freedom Caucus member Rep. Justin Amash returned fire in the Republican civil war:

"It didn't take long for the swamp to drain @realDonaldTrump. No shame, Mr. President. Almost everyone succumbs to the D.C. Establishment," Amash wrote.

It didn't take long for the swamp to drain @realDonaldTrump. No shame, Mr. President. Almost everyone succumbs to the D.C. Establishment. https://t.co/9bDo8yzH7I — Justin Amash (@justinamash) March 30, 2017

Later, Amash told Fox News that "most people don't take well to being bullied" and compared Trump's tactics to those of a fifth grader.

Freedom Caucus co-founder Jim Jordan, however, refused to take Trump's bait during an interview on "America's Newsroom."

"We appreciate the president," Jordan said. "We're trying to help the president, but the fact is you have to look at the legislation."

He added: "I'm not here to assign blame to anyone…what I focus on doing is doing what I told the voters we're going to do."

The 30-plus member Freedom Caucus in the House of Representatives was the key bloc that refused to vote in favor of the Trump-backed health care bill earlier this month. Despite the bill’s general lack of popularity, the consensus was that it could have been able to pass the Republican-majority House – if the Freedom Caucus joined in.

But Trump and top advisers were never able to allay the concerns of caucus members, and House Speaker Paul Ryan pulled the bill – which would have been Trump’s first major legislative achievement and dealt a death blow to ObamaCare – prior to a scheduled Friday vote.

Ryan said during a Thursday news conference that he understood "the president's frustration," regarding Trump's tweet.

Trump hasn’t taken health care reform off the table entirely in the aftermath of the debacle; however, he and his aides have moved on to other issues such as tax reform and infrastructure.

Trump’s tweet suggests he may attempt to primary some members of the caucus in 2018, hoping to find a candidate more favorable to his agenda. It’s a tactic he’s threatened in the past, notably in the cases of former presidential primary rivals Sen. Ted Cruz, R-Texas, and Ohio Gov. John Kasich, who had refused to endorse Trump. Cruz eventually gave his endorsement.

The Republican infighting has not been limited to the House.

Ryan and a top Senate Republican engaged in a brief public spat Thursday about comments Ryan made earlier in the morning, seeming to suggest Trump should not try to work with Democrats.

“What I worry about, Norah, is that if we don't do this, then he'll just go work with Democrats to try and change ObamaCare and that’s not – that’s hardly a conservative thing,” Ryan told CBS.

Sen. Bob Corker, an avid backer of Trump's during the presidential campaign who was among those considered to be vice president, shot back on Twitter: "We have come a long way in our country when the speaker of one party urges a president NOT to work with the other party to solve a problem."

Ryan, during his news conference, dismissed Corker's remarks.

"They're not going to help us repeal ObamaCare, that's my point," Ryan said of Democratic lawmakers.