Republicans should act swiftly to withdraw their support of Donald Trump and find a replacement for the top of their presidential ticket, MSNBC's Joe Scarborough argued in an op-ed on Wednesday.

"A bloody line has been crossed that cannot be ignored," the former Republican congressman wrote in the Washington Post. "At long last, Donald Trump has left the Republican Party few options but to act decisively and get this political train wreck off the tracks before something terrible happens."

Scarborough, who has evolved into one of Trump's greatest conservative critics after routinely defending him in the primary, claimed the GOP nominee's latest remark about "Second Amendment people" stopping Hillary Clinton from nominating federal judges should send a clear message to Republicans: They "must dump Trump."

"The Secret Service should interview Donald Trump and ask him to explain his threatening comments," he wrote. "[House Speaker] Paul Ryan and every Republican leader should denounce in the strongest terms their GOP nominee suggesting conservatives could find the Supreme Court more favorable to their desires if his political rival was assassinated."

Scarborough added that Republican leaders need "to start examining quickly their options for removing [their] nominee."

Ryan told reporters that Trump's comments sounded "like a joke gone bad," but the House speaker pushed back when a Latino worker peppered him about dumping Trump during a Wisconsin manufacturing tour on Tuesday.

"He won the votes fair and square, he won more votes than anybody else ... so he is the nominee," Ryan said of Trump. "He got 14 million votes, and nobody else got close to that. So that's just the way the system works."