Republicans enter the fall campaign in moods ranging from grim foreboding to howling despair. They fear that Donald Trump will not only lose but lose so big he will take hordes of other candidates down with him, costing the GOP control of the U.S. Senate and even the House. This election could be the party’s worst debacle since 1964.

Republicans don’t seem to have prepared for an even bigger catastrophe that could occur Nov. 8: a Trump victory. In that case, they wouldn’t be stuck with him for the next two months. They would be stuck with him for the duration of his presidency, and they would have to answer for him forever.

They are in the position of a bride who, on the eve of her wedding day, knows she’s making a mistake.

If she backs out, she’ll bring a mess down on her head. But if she doesn’t, she’ll be caught in a snare that will be painful and hard to escape, with consequences she will have years to regret.

The first harm from Trump is that he would be the new identity of the party. Forget the legacy of Ronald Reagan. Never mind what Mitch McConnell and Paul Ryan propose. He would be the one defining the national agenda.

If President Trump wanted to intern Muslims, launch drones against Mexico or put David Duke up in the Lincoln Bedroom, his fellow Republicans would wear the stain.

One of the miseries they have suffered in recent months is waking up each day anxiously wondering what new folly their candidate is about to commit. It’s bad enough having to put up with his insulting of a gold star mother, not knowing that Russia has invaded Ukraine, accusing Barack Obama of founding the Islamic State, and retweeting white supremacists.

But all this amounts to an ignorant egomaniac running his mouth. In the White House, Trump would be acting, not just talking. He would possess powers that can be wielded in all sorts of destructive ways. As Republicans have learned from Obama’s use of executive authority, it’s hard to stop a determined president from doing whatever he damn well pleases.

Scrap NAFTA? Carry out indiscriminate bombing of the Islamic State? Refuse to come to the aid of a NATO ally attacked by Russia? Bring back torture, using methods that would make Dick Cheney weep?

Turn over decisions to advisers who couldn’t find their way out of an elevator if you gave them a map and a compass? Dump Melania and start dating? The question is not whether Trump would make bad choices in the White House — only which ones and when.

Since he wrapped up the nomination, Republicans have been hoping Trump would change his reckless style, listen to people who know more than he does, avoid pointless fights and generally behave like a responsible adult. Their hopes have been in vain. He either can’t change or sees no reason to.

Winning the election would turbocharge Trump’s worst impulses. He would have new grounds to ignore GOP leaders and indulge his every whim. If that approach gets him elected, why would he behave any differently as president?

Maybe Trump would drag the country through four years of chaos and stagnation, trailing broken promises and aborted schemes.

Or maybe he would handle the job so irresponsibly that he would provoke his impeachment and removal — an eminently plausible scenario.

The latter outcome would have some special downsides for Republicans. One is that it would saddle them with the herculean chore of defending him at his worst. Another is that it would derail any policy ideas they hope to advance. Then there’s the political cost in the next election.

Compared with these nightmares, a Hillary Clinton presidency would have all sorts of advantages. It would give Republicans a unifying focus, mobilize them to block liberal policies, open the way for new conservative leaders to emerge and offer the party a chance to rebound at the polls in 2018. If she were to be embroiled in a White House scandal brought on by her own disregard for the rules, even better for the GOP.

Republicans might remember British statesman Benjamin Disraeli’s explanation of the difference between a misfortune and a calamity. For his chief rival to fall into the river, he said, would be a misfortune. The calamity would be if someone pulled him out.