Some members, particularly those in tough re-election fights, are electing to stay vague and elliptical. When asked in a local radio interview Monday about Mr. Trump’s initial reluctance to denounce Mr. Duke, Senator Ron Johnson, Republican of Wisconsin, said more than once that he “prays every night” that the Republican presidential nominee “is a person of integrity, intelligence, ideas, and courage.”

For other members, the balance of the Supreme Court remains a key issue, and on that logic alone they are resisting being too critical of the potential nominee, no matter his flaws.

“I’d rather be in our shoes than in Hillary Clinton’s shoes any day of the week,” Mr. Collins said.

Worried about the prospects of a negative Trump effect on some House races — even though gerrymandered districts make it exceedingly unlikely for Republicans to lose the House — Kevin McCarthy of California, the House majority leader, is advising members to focus on local issues and priorities in their respective districts.