He was already being trounced in primary polls and has expressed interest in running for president as a third party.
Via Fox News:
Michigan Rep. Justin Amash announced Thursday that he is leaving the Republican Party, saying he is disenchanted and “frightened” by party politics — a move that was quickly described as “great news” by President Trump.
Amash, a libertarian and a critic of President Trump, made the announcement in an op-ed for The Washington Post, in which he said he had once run as a Republican due to the GOP’s belief in limited government, economic freedom and individual liberty.
“In recent years, though, I’ve become disenchanted with party politics and frightened by what I see from it,” he said in the op-ed. “The two-party system has evolved into an existential threat to American principles and institutions.”
Trump soon responded, calling Amash’s departure “great news” for the GOP and suggesting he quit because he was likely to lose to a primary challenge for his re-election bid.