The White House is ramping up its public battle with the ultraconservative House Freedom Caucus, singling out Rep. Justin Amash (R-Mich.), one of the group’s most unflinching members.

President Donald Trump’s social media director Dan Scavino first needled Amash on Friday, tweeting that the Great Lakes State congressman refused to back the Republican Obamacare replacement bill because “he puts politics before [Michigan.]” Scavino included screenshots of articles about the congressman’s decision not to vote for Trump, and of an Amash tweet criticizing the president’s rants about Rep. John Lewis (D-Ga.) in January.

Just saw Never Trump Freedom Caucus Troll @JustinAmash {MICHIGAN} comments re: @POTUS. Didn't vote for #AHCA b/c he puts politics before MI. pic.twitter.com/xQqv1FhFiq — Dan Scavino Jr. (@DanScavino) March 31, 2017

Amash, known for his stubborn independence from party leadership, was defiant as ever, defending opposition to the bill he called “#Swampcare because it’s just another version of #Obamacare.”

Didn't vote for #Swampcare because it's just another version of #Obamacare—and just as dysfunctional (which is pretty hard to pull off). https://t.co/qSYB1AO6BP — Justin Amash (@justinamash) March 31, 2017

Scavino continued hammering Amash on Saturday, calling for Trump supporters to primary Amash in the coming election cycle.

.@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

Amash responded in less than an hour, claiming the Trump administration and establishment “have merged into #Trumpstablishment.”

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

If the White House is serious about ousting Amash, it will not be easy. A more mainstream Republican backed by the party establishment tried to defeat the four-term congressman in a 2014 primary and failed miserably.

Amash has emerged as the congressional Republican perhaps most willing to criticize Trump. He has maintained this maverick approach during the public feud that has erupted between Trump and the Freedom Caucus in the past few days.

Tom Williams/Getty Images Rep. Justin Amash (R-Mich.), one of the House Freedom Caucus' staunchest members, is now the target of Twitter attacks by President Donald Trump's social media director.

The president has settled on a deliberate strategy of calling out the Freedom Caucus for its role in sinking the Republican Obamacare replacement bill. The group of hardline fiscal conservatives argued that the legislation preserved too much of the Affordable Care Act’s original structure, noting that it would provide tax credits for individuals to purchase insurance on exchanges.

Trump tweeted on Thursday that Republicans “must fight” Freedom Caucus members if they “don’t get on board,” implying that they would be subject to primary challenges. On Friday, he singled out GOP Reps. Mark Meadows (N.C.), Jim Jordan (Ohio) and Raul Labrador (Idaho) ― all Freedom Caucus members ― in more conciliatory tweets that seemed aimed at cajoling them into cooperation.

Scavino’s call to initiate a primary challenge against Amash is the most explicit indication yet that the White House plans to try to push these members out if they stand in the way of the broader GOP agenda.