WASHINGTON — President Trump attacked Representative Justin Amash as a “total lightweight” and “loser” on Sunday, a day after the Michigan Republican said Mr. Trump’s behavior as president had reached the “threshold for impeachment.”

The president’s attacks reinforced Mr. Amash’s isolation within his party, as even the Republican lawmakers who might be most sympathetic to his position avoided stepping forward to join him.

Earlier on Sunday, Senator Mitt Romney, a Utah Republican who has been one of the few members of his party to even mildly chastise Mr. Trump in public after the release of the Mueller report, described Mr. Amash’s statement as “courageous.” But Mr. Romney, the 2012 Republican presidential nominee, dismissed the idea of impeachment, saying on CNN’s “State of the Union” that the evidence lacked “the full element that you need to prove an obstruction-of-justice case.”

Mr. Trump — who has stonewalled requests by House Democrats for documents and has commanded current and former aides to turn down requests to testify before investigative committees — was not so circumspect.