Revisiting the now hollow critiques from Mr. Trump’s own party — those Republicans have all embraced the president and his agenda, going on to serve in his administration or offer him support from Capitol Hill — is raising questions about whether the search for a killer line on Mr. Trump is a fool’s errand.

“Voters on the whole are completely desensitized to personal attacks on Trump at this point,” said Tim Alberta, the author of “American Carnage: On the Front Lines of the Republican Civil War and the Rise of President Trump.”

“Rick Perry called him a ‘cancer’ and then became a cabinet secretary,” he said. “It’s not like a swing voter in a battleground state will hear an ad hominem attack on him and suddenly think, ‘I never thought of it that way.’”

Mr. Alberta, who in researching his book spent time Monday-morning quarterbacking the 2016 election with many of the failed Republican candidates, said that “the unlearned lesson from 2016 for Republicans was that every day spent launching ad hominem attacks on Trump was a day not spent pointing to voters how little he knows about actually running the government.”