The parties have reversed field since the time when Bill Clinton could say that, in presidential politics, Republicans fell in line and Democrats fell in love.

Republicans — or at least a significant element of the party — are utterly besotted with Trump, while Democratic officialdom is dutifully lining up behind Biden despite polling showing that the rank-and-file has deep misgivings about him.

The Iowa-Iowa State football game last weekend showed Trump at full force. He got his share of boos, but the throng of people who lined up to see him walk along a corridor at the stadium was ecstatic and adoring.

Eight years after coming down the escalator, Trump still generates an electric response among his supporters that’s rare to see in our politics. The only thing like it in recent memory is the reaction that Barack Obama generated back when he had people routinely fainting in the huge crowds standing for long stretches to see him.

For his fans, Trump is a hero, a celebrity, and a symbol — of a forceful rejection of the premises of the woke left, of perseverance despite fierce political opposition, and of a certain muscular Americanism. There’s a reason his supporters spontaneously chant “USA!” upon seeing him. Lenin said of the Romanovs that they were living flags, which is why they had to be liquidated to make way for the new regime. For his devotees, Trump, too, is a living flag.

There is no advance team or door-knocking-operation that could gin up this sort of enthusiasm. Trump is a phenomenon — larger than life, a legend — whereas his opponents are mere political candidates. For non-Trump Republicans, it’s a little like trying to take down the Beatles with Rick Springfield or Debbie Gibson.

All of this makes Trump difficult to beat, but, obviously, not necessarily a good nominee or choice to be the next president of the United States.

It’d behoove Republicans to be less enamored and more cold-eyed. That they are entertained by Trump and feel defensive of him does not wipe away his deficiencies.

It doesn’t require much foresight to realize what next year could be like if Trump is nominated, with trials and convictions perhaps in the offing and a relentless Democratic focus on all the vulnerabilities — Jan. 6, various and sundry wild-eyed statements — that no other Republican possesses. Even if Trump wins the presidency again, his administration could well look more like the shambolic end of his first term in office rather than the earlier stretches when he had impressive people working for him and things were (relatively) under control.

Why court all of this when there are other acceptable options who’d have better odds against Joe Biden and who’d have a better chance of running effective presidencies? Well, to quote Emily Dickinson or Selena Gomez depending on your tastes, the heart wants what it wants.

If another Republican somehow prevails over Trump for the nomination, it will be analogous to how Biden defeated Trump in 2020, with every outward measure of enthusiasm clearly against him.

The Democrats are swinging the opposite way. They are with Biden in 2024, not because anyone feels inspired to mount the ramparts for him, not because he has any hope of attracting adoring throngs or even lukewarm throngs, but due to a bloodless calculation that Joe Biden is best suited to beat Donald Trump.

Biden isn’t a symbol or a cause, rather an instrument. If Trump represents a politics of some sort of madcap poetry, Biden represents a politics of dreary, “uh, let me see who it says here I should call on next” prose. Trump is the candidate of mass politics in the social media age, whereas Biden has more the feel of a candidate of smoked-filled rooms, even if such rooms no longer exist.

There’s nothing inherently wrong with that. Fervor in politics isn’t necessarily a good thing. The problem is that the thoroughly practical bet on Biden is not nearly as practical as Democrats believe.

Biden is not the only Democrat who can beat Trump, and doesn’t have the best chance of beating Trump. To the contrary, Biden and his running mate Kamala Harris probably match-up against Trump more poorly than other plausible national Democratic candidates.

Democrats are not betting on Barack Obama, who simply had more political talent than his opponents, or Bill Clinton, a charmer with boundless energy who could talk his way out of anything. On his best days, Biden is pedestrian, and he’s having more bad days.

Democrats who watched the president’s recent Vietnam press conference and didn’t realize that Biden could easily find a way to lose to Trump are fooling themselves.

Yes, Biden beat Trump once before, but it was a narrow victory in the key swing states — prior to Biden becoming an unpopular incumbent with low ratings on the economy whose advanced age has become a matter of near-universal concern.

Could that be enough to tip another close race the other way? Of course. The wager on Biden is essentially the hope that Trump’s weaknesses will outweigh Biden’s weaknesses. That may prove the case, but wouldn’t it make more sense to nominate a candidate with fewer weaknesses?

Assuming there was a way to get Biden to not run in 2024, Democrats don’t want to try to nominate a stronger candidate out of fear — fear that the primary process would be chaotic and divisive; fear that a candidate to Biden’s left would win the nomination; fear, specifically, that the unpopular Harris would win, and perhaps fear, as well, that she would lose, alienating African-American women.

So, there you have it. Democrats hold tight to nurse for fear of something worse, while smitten Republicans fall into the arms of their champion — and most American voters get the rematch that they dread.