All this is playing out against the backdrop of midterm elections, where lawmakers will face Republican voters who are still wildly enthusiastic about Mr. Trump and have, in many cases, adopted his skepticism about the Russian interference. Attacks by Mr. Trump and his allies on Capitol Hill and Fox News against those investigating him have not only fired up the president’s base but, polls show, substantially eroded trust in the impartiality of the special counsel, Robert S. Mueller III, and the F.B.I. itself.

Some Republicans have concluded that keeping their heads down without uttering much more than general statements about Russian hostility is the only safe course.

“There’s nothing you can do to stop a president, let alone this president, from saying what he thinks or what he wants to say,” said Representative Ryan Costello, Republican of Pennsylvania, who broke with party leaders to endorse proposed legislation protecting Mr. Mueller’s job. “I think a lot of Republicans feel it’s not worth engaging because all you do is upset a lot of Republican voters.”

Democrats view Russia’s election interference as nothing short of an existential threat to American democracy, and have repeatedly pushed Republican leaders to take a tougher line toward Mr. Trump and stop the attacks on investigators.

“The road to the Helsinki disaster was paved by Republican inaction every time Trump overstepped,” said Senator Chuck Schumer of New York, the Democratic leader. “Their silence, their acquiescence to things they know are wrong have given Trump the extra jolt he needed.”

Even before Mr. Trump was elected, Democrats and Republicans grappled with how to respond as Russians were hacking and leaking Democratic emails, flooding social media with pro-Trump and anti-Hillary Clinton messages, and even organizing pro-Trump rallies. In September 2016, President Barack Obama summoned congressional leaders to the Oval Office to ask them to issue a strongly-worded bipartisan letter to state and local officials raising alarms about the Russian threat.