Former President George W. Bush won’t support the re-election of Mr. Trump, and Jeb Bush isn’t sure how he’ll vote, say people familiar with their thinking. Senator Mitt Romney of Utah won’t back Mr. Trump and is deliberating whether to again write in his wife, Ann, or cast another ballot this November. And Cindy McCain, the widow of Senator John McCain, is almost certain to support Mr. Biden but is unsure how public to be about it because one of her sons is eying a run for office…

Former Republican leaders like the former Speakers Paul D. Ryan and John A. Boehner won’t say how they will vote, and some Republicans who are already disinclined to support Mr. Trump are weighing whether to go beyond backing a third-party contender to openly endorse Mr. Biden. Retired military leaders, who have guarded their private political views, are increasingly voicing their unease about the president’s leadership but are unsure whether to embrace his opponent…

“I’ve had five conversations with senators who tell me they are really struggling with supporting Trump,” said Mr. Coons, who declined to give names.

Indeed, one Republican senator, who is publicly supporting the president, said in an interview that he might prefer a Biden victory if the G.O.P. managed to preserve its Senate majority. This lawmaker, like a number of Republicans, is uneasy with Mr. Trump’s behavior and weary from the near-weekly barrage of questions from reporters about the latest presidential eruption.