Conservative critiques of Mr. Trump, from senators such as Jeff Flake of Arizona and Ben Sasse of Nebraska, have gone beyond policy disagreements, to temperament, defenses of democratic institutions and law enforcement, and the conduct of foreign policy. And other Republican leaders are speaking out. John Cornyn of Texas, the No. 2 Republican in the Senate, is giving almost daily speeches on the Senate floor pushing for action on a modest but bipartisan bill to fortify the existing background check system for gun purchases.

Senator Orrin G. Hatch of Utah, the chairman of the Finance Committee and a loud supporter of Mr. Trump, nonetheless scorched his tariffs as “a tax hike the American people don’t need and can’t afford.”

Mr. Ryan has repeatedly urged Mr. Trump to abandon the tariffs in private, according to one person familiar with their conversations. In public, he has held his tongue. He declined to be interviewed.

“I always imagined that there was room for an alternative conservative vision to Trumpism, and Paul Ryan was ideally suited to be that leader of a non-Trumpist conservative Republican Party, but he’s chosen not to assume that role,” Mr. Sykes said. “It’s very disappointing to see he’s become so comfortable as an ally and an enabler of Trump.”

For Democrats, the speaker’s deference to Mr. Trump is the stuff of ridicule — and a constitutional dereliction of duty by the man three steps from the presidency.

“Can you imagine Tip O’Neill or Sam Rayburn or any of the portraits in this gallery saying, ‘I don’t know whether I can be for that, I’ve got to go down the street and ask permission of the president’?” asked Representative Anna G. Eshoo, Democrat of California, referring to previous speakers whose portraits hang just outside the House chamber.

To be sure, Mr. Ryan has had some achievements, notably the rewrite of the federal tax code, which for the speaker marked the realization of a long-held dream. He pushed a repeal of the Affordable Care Act through the House, only to see it die in the Senate. And he helped negotiate a two-year budget deal that should bring some peace to Washington even as it balloons federal spending.