“Instead we’re relying on somebody who’s tweeting with exclamation points,” said Mr. Walsh, referring to Donald J. Trump’s initial response to Mr. Comey’s news conference…

For many in the party it also was a painful reminder of what could have been — how a different standard-bearer could have capitalized on one of the most difficult days Mrs. Clinton has faced as a candidate. For the Republican establishment, the months since Mr. Trump began closing in on the presidential nomination have been a season of dismay and frustration: Handed a historically weak Democratic opponent to run against, the party’s voters responded by nominating a candidate even more unpopular and toxic than Mrs. Clinton.

But there have been few days during this cycle of disbelief in which the sense of regret has been as palpable for Republican strategists and policy makers as when Mr. Comey jolted the political world back to life after a long holiday weekend.

“Imagine Jeb Bush looking disappointed and talking about the importance of following the rules and a society ruled by law with a government that is held accountable,” said Kori Schake, a fellow at Stanford’s Hoover Institution and a national security aide in George W. Bush’s administration who is now backing Mrs. Clinton. “This should be a really great moment for a Republican nominee. But there’s no way in the world Donald Trump could pull that off.”