The Trump administration’s public feud with the hardline conservative House Freedom Caucus escalated Saturday with a top White House aide calling for a primary challenge for Rep. Justin Amash (R-MI)—a prominent and outspoken member of the group who opposed the ill-fated bill to repeal the Affordable Care Act.

Dan Scavino Jr., the social media director and senior adviser for the Trump White House, called out Amash by name on Twitter, referring to the congressman a “big liability” and urging someone to step forward to unseat him in 2018.

.@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

Legal experts, including former White House lawyer Daniel Jacobson, say Scavino may have violated the Hatch Act with this tweet. The 1939 law explicitly forbids White House staff from using their “official authority or influence for the purpose of interfering with an election or affecting the result thereof.”

Amash, who has already survived several primary challenges from establishment Republicans in the past, has also been singled out by Trump himself, who recently vowed to “fight” both the Freedom Caucus and Democrats in future elections.

Amash told the Washington Post that these public attacks, on social media and in the press, would only be “constructive in fifth grade.”

“It may allow a child to get his way, but that’s not how our government works,” he said.

Amash has good reason to be confident. In his Michigan district he ran far ahead of Trump, winning 60 percent of the vote in 2016 compared to Trump’s 52 percent.

He fired back on Twitter, saying Trump has become the very Establishment he campaigned against in 2016.

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

By mid-afternoon, Amash was raising funds off of the White House threat to primary him.