Donald Trump doesn't handle setbacks all that well.

Take that time when he lost the Iowa caucuses to Ted Cruz in the 2016 presidential campaign. The next  day, Trump insisted that something nefarious had taken place.

“Ted Cruz didn't win Iowa, he illegally stole it,” Trump tweeted. “That is why all of the polls were so wrong and why he got more votes than anticipated. Bad!”

Or that time when he won the electoral college over Hillary Clinton but lost the popular vote by more than 3 million.

Trump not only hates losing but also seems to refuse to accept it — either denying the loss entirely or putting the blame on someone (anyone) other than himself.

Which brings me to tomorrow's much-anticipated vote on the first stage of Trump's efforts to reform the Affordable Care Act.  Trump has made it very clear that he is all in on ensuring that the plan gets to 216 votes in the House — and has followed through with repeated visits with wavering Republican members of Congress to cajole them to be for the bill. (Trump headlined a National Republican Congressional Committee dinner Tuesday night, for example.)

And yet, most whip counts — including The Post's — suggest that if the vote happened today, the bill would fail, albeit narrowly.

Trump — and White House press secretary Sean Spicer — have been careful to focus on the carrot, not the stick for Republican members who are against them. Sure, Trump joked with House Freedom Caucus chair Mark Meadows that he would “come after” him if he voted “no” but, by and large, the president has tried to emphasize the positives of voting for the bill — including massive (and unlikely) seat gains.

“One of the things that he made clear this morning was that he was going to make sure that the people who did support this, he would be out there supporting them,” Spicer told reporters during his Tuesday news conference. “And so I’m not going to focus on the negative as much as the positive today.”

But it's also clear in Trump's “joke” to Meadows or Spicer's assertion that the members who vote against the bill will “probably pay a price at home” that menace lies just below the surface here. Trump has never demonstrated a willingness or an ability to take the high road when faced with adversity. He views dissent as disloyalty — and he HATES disloyalty.

The most likely reaction from Trump if the Republican-controlled House votes down the health-care proposal Thursday night then is anger — and payback. Trump is vengeful as a politician — his ouster of Chris Christie, his shabby treatment of Mitt Romney post-election — and believes it's central to his brand and his effectiveness. People need to know that when they act in ways he doesn't approve of, there are real consequences. Not like, “Hey, I wish you hadn't done that” consequences, either. Like, beat-you-in-an-election consequences.

If Trump goes full napalm on Republicans in the event of a failed health-care vote, it probably dooms any chance of taking another bite at that apple. Which means that Republicans will head into the midterm elections having not done the one thing they promised to do for the past decade if they somehow got control of all levers of power in Washington.

I'm not sure Trump would care. He would likely view those members as getting what they deserved for refusing to be for something he wanted. And he might even conclude that an election bashing in 2018 would help convince congressional Republicans that being for him is the only way for them to survive.

There is, of course, the much smaller possibility that Trump plays good cop in the wake of a failed vote and works with House Speaker Paul D. Ryan and Senate Majority Leader Mitch McConnell to cobble together some other health-care proposal that would give Republicans on the ballot in 2018 something to take to their constituents.

But everything we know about Trump suggests the former option is WAY more likely than the latter. Would it create chaos for Republicans if Trump took that path? Yes. But he likes chaos.