A White House aide made an unusual move Saturday by publicly urging voters to oppose a congressman from President Donald Trump's own party.

Dan Scavino Jr., director of social media for the White House, called on Michigan voters to "defeat" Rep. Justin Amash, a Republican House Freedom Caucus member and a frequent Trump critic.

Scavino called the fourth-term congressman a "liability" in the state, which Trump unexpectedly won in November, partly driven by promises to boost manufacturing there.

.@realDonaldTrump is bringing auto plants & jobs back to Michigan. @justinamash is a big liability.#TrumpTrain, defeat him in primary.

The libertarian-leaning Amash was among the conservative and moderate Republicans who contributed to defeating the GOP plan to replace the Affordable Care Act.

At least for now, Amash—among the Congressional Republicans most critical of Trump —appeared unbowed by Scavino's challenge. He responded to Scavino in a tweet, saying the White House aide was attacking "independent thinkers" and used the "same old agenda" of "establishment" politicians.

Trump admin & Establishment have merged into #Trumpstablishment. Same old agenda: Attack conservatives, libertarians & independent thinkers.

The back-and-forth escalates tensions between Trump and members of his party, whose support he may need as Republicans try to pass a tax reform plan in the coming months. On Thursday, Trump called out specific Freedom Caucus members on Twitter, vowing to "fight" the group in 2018 and arguing that they threatened to hold up the GOP agenda.

It is not clear if Trump's attacks will make reluctant members of Congress more likely to follow his lead, especially because his approval rating sits well lower than recent presidents at this point in his administration.

Amash is known for explaining his votes to constituents and reportedly cried this year when he missed his first vote since taking office in 2011. Days ago, Amash likened the president to a 5th grader for his broadside against the Freedom Caucus.