An aide to President Donald Trump called for Michigan Republican Rep. Justin Amash, a member of the conservative House Freedom Caucus (HFC) and outspoken critic of the president, to be challenged and defeated in the 2018 congressional primary.

Dan Scavino Jr., White House director of social media and senior aide to Mr. Trump, labeled Amash a “big liability” in a tweet Saturday, urging followers to “defeat him in primary.”

.@realDonaldTrump is bringing auto plants & jobs back to Michigan. @justinamash is a big liability.#TrumpTrain, defeat him in primary. — Dan Scavino Jr. (@DanScavino) April 1, 2017

Scavino’s tweet was the latest White House attack on HFC members who helped derail the Republican health care bill, after Mr. Trump threatened in a tweet Thursday morning to oppose the conservative Republicans in their 2018 primaries.

Amash fired back quickly on Twitter, saying Mr. Trump and the establishment have become one and the same. He also tweeted a link to his fundraising page, encouraging challengers to “bring it on.”

Trump admin & Establishment have merged into #Trumpstablishment. Same old agenda: Attack conservatives, libertarians & independent thinkers. https://t.co/ALcV59iHXx — Justin Amash (@justinamash) April 1, 2017

Bring it on. I'll always stand up for liberty, the Constitution & Americans of every background. You can help here: https://t.co/ECxQGXE5XH. — Justin Amash (@justinamash) April 1, 2017

Amash, a conservative with libertarian positions on privacy and Fourth Amendment issues, has consistently been one of the most vocal Republican critics of Mr. Trump. Amash was one of the first Freedom Caucus members to respond to Mr. Trump’s threat to oppose the conservative Republicans in 2018, tweeting that the president has succumbed to the “D.C. establishment.”

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

He went on to call Mr. Trump a “childish bully,” saying that his attacks would be “constructive in the fifth grade. It may allow a child to get his way, but that’s not how our government works.”

Get Breaking News Delivered to Your Inbox

Amash has been criticizing Mr. Trump for months, so much so that the Huffington Post profiled him in a December piece called, “The one House Republican who can’t stop criticizing Donald Trump.”

“I’m not here to represent a particular political party; I’m here to represent all of my constituents and to follow the Constitution,” Amash told the Huffington Post at the time.

Mr. Trump’s White House continues to fuel a divide among the Republican caucus that has only become more strained since his inauguration. Recently, Mr. Trump has turned his frustration on the limited government-minded HFC, whose members have generally tried to balance support with reservations about the president.