Hillary Clinton makes a stop at the Lincoln Square pancake house in Indianapolis on May 1. (Photo: Joe Raedle/Getty Images)

If you want to experience the full-on contempt of the leftist intelligentsia right now, go on social media and suggest, as I did this week, that Donald Trump isn’t certain to get crushed in November. (Trump, in case you hadn’t noticed, brings out pretty much the worst in everybody.)

The way a lot of partisan Democrats see it, Hillary Clinton — despite a loss to Bernie Sanders in Indiana Tuesday — will soon lock down her party’s nomination, and the only way she finds herself even threatened by Trump is if the media decides to legitimize him so we all have something to talk about. The word I keep hearing from liberals is “layup.”

Clinton does, in fact, enter the general election season with some serious structural advantages. Having analyzed trends from the past six elections and factored in demographic shifts, Third Way, the leading centrist Democratic group, concluded that Clinton starts the campaign virtually assured of 237 electoral votes — 46 more than Trump and just 33 short of a majority.

And as you’ve probably heard, no candidate has ever overcome — or even tried to overcome — the kind of ugly impressions Trump has made on women and minority voters to this point. Next to him, Clinton polls like Santa Claus.

But if history is any guide, Clinton comes to the campaign with a structural disadvantage, too, and one that shouldn’t be overlooked. It may explain why she can’t seem to put Bernie Sanders away — and why the outcome in November is hardly assured.

I’ve gone through this history once or twice before, but it bears repeating: In 1947, Congress passed the 22nd Amendment, which said no one could be elected to the presidency more than twice.

In the 65 years since the last state ratified that amendment — comprising 16 elections, and six elections following an eight-year presidency — only one nominee has managed to win a third consecutive term for his party. That was George H.W. Bush, who overcame a double-digit deficit late in the campaign, thanks in part to one of the most ineffectual Democratic campaigns in history.

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(And before you start with me, I know, Al Gore actually won, and in an alternate universe somewhere they are building his monument on the Tidal Basin in a climate that is, on average, four degrees cooler than the one we inhabit, but for purposes of this discussion, let’s just live in the here and now.)

The important question is why it’s proved so difficult for either side to win third terms. The most common explanation has to do with voter fatigue. Essentially, we’re told that voters get sick of having one party in office for eight years, and so the pendulum swings back.

I don’t find this theory especially persuasive. I’ve met an awful lot of voters over the years, and rarely have I heard anyone make the case that it was time for the other party to get a turn. It seems to me voters focus a lot more on the candidates themselves than on the parties they represent.

And this may get to the truer cause of the third-term conundrum. If you look back at elections over the past half century, what you find is that the parties of two-term incumbents almost always nominate the candidate who is nominally next in line. Of the six candidates who have sought third terms since 1960, five had previously served as either president or vice president. (The president was Gerald Ford, who ran for election in 1976 after having held the job for two-plus years.)

The outlier was John McCain, who, like Clinton, had been the runner-up in the last open election, and who ran in a year when the incumbent vice president was sitting it out.

It’s not hard to see how this happens. A two-term president has both the time and the muscle to set up someone who will carry on his legacy — while effectively boxing out challengers.

And because presidents almost always lose congressional seats and governorships in off-year elections, an eight-year presidency tends to decimate the ranks of worthy, younger successors from outside the establishment, anyway.

In other words, by the time a president gets done slogging his way through the peaks and troughs of eight years on the job, there aren’t a lot of new, exciting alternatives to whichever former rival or loyal No. 2 has been patiently waiting on the edge of the stage.

The problem is that the next-in-line — Gore, McCain, Hubert Humphrey, Richard Nixon in 1960 — is almost never as politically gifted as the president he (or she) has served. If he were, he wouldn’t have ended up next-in-line to begin with.

And that inferiority is only magnified by a dilemma that even the best politicians would find damn near impossible to navigate. The next-in-line has to be loyal without being small, embody the future while representing the past. He has to somehow embrace continuity while at the same time putting distance between himself and the inevitable disappointment a president leaves behind.

The next-in-line always has more trouble than he should unifying the party, because the fissures that were suppressed through eight years of a presidency — in the cause of staving off the opposition — rise to the surface. The end of every eight-year presidency is something like the fall of Tito, with disparate factions and pent-up emotions finally unleashed.

Clinton — runner-up in 2008, loyal soldier thereafter — is the prototypical next-in-line. Thanks to a couple of dreadful midterm election cycles, she’s had to contend only with a 74-year-old protest candidate who just recently joined the party, and even then she hasn’t been able to excite enough of her own party’s base to lock down the nomination by May.

She’s had to lash herself tightly to the president while at the same time trying to co-opt the ideological fury among the party’s dissatisfied factions. She will emerge from this process with her agenda opaque, her convictions hedged.

Maybe Gore and McCain, having gone through the exact same thing, have some sort of support group she can visit.

Unlike both of those guys, of course, Clinton seems to have gotten astoundingly lucky in her opposition. It’s true: Trump’s appalling rhetoric will make for some whopping TV ads. And yes, if his numbers hold, especially among women, Trump’s next reality-show gig might be called “The Biggest Loser of All Time.”

But here’s the thing about Trump: He’s run the flat-out most offensive, least substantive and crassest campaign in memory, and national polls show him trailing Clinton by 10 points, with six months yet to go.

Think about that. In presidential politics, 10 points can fall away faster than Carly Fiorina on a riser.

And while voters’ impressions at this point in a campaign are normally hard to change, what we don’t know about Trump — the big question, to my mind — is whether the larger electorate will ultimately judge him by the standards of a politician or, like primary voters, as a celebrity.

Politicians aren’t allowed to simply shrug off their records and respawn entirely. The voters, finely attuned to any sign of inauthenticity, won’t have it.

But entertainers reinvent and redeem themselves all the time; it’s what gossipy magazines exist for. And Trump is closer to inhabiting this realm than any candidate we’ve ever seen.

Don’t expect the Trump who takes the stage in Cleveland to be remotely like the Trump who bragged about his genitalia in a debate. And don’t assume, just because his bigotry and base antics are a matter of record, that the rules of traditional politics will apply.

Next-in-lines have been known (at least once) to win, and assuming she can nail down the nomination, Clinton is as clear a favorite as we’ve seen in a while. But Clinton shouldn’t delude herself into thinking she’s headed for a layup, and neither should anyone else.