It’s a familiar plotline. An interloper runs for a party’s nomination and, with an anti-insider pitch, scores wins and near-wins in the first contests with vote pluralities. His numerous opponents, fearful of antagonizing his enthusiastic supporters, launch attacks on one another that, predictably, hurt the attacker as well as the target.

Party establishment types, convinced the interloper is a sure loser in November, dither and tilt things mildly against him while trying to maintain the impression of fairness. Candidates with no chance remain in the race, dividing the anti-interloper vote, hoping the interloper will collapse. But he doesn’t — and even starts winning absolute majorities in late contests.

That’s how Donald Trump won the GOP nomination in 2016, and so far, it’s consistent with how Bernie Sanders is doing in the 2020 race.

Remember that Trump lost the Iowa caucuses to Ted Cruz, and that 65 percent of New Hampshire Republicans voted for somebody else. Sanders’ performance so far is comparable: He had a lead in first- and second-round popular votes in Iowa, though he lost state delegate equivalents to Pete Buttigieg for a total of 564-562; and he won 26 percent to 24 percent over Buttigieg in New Hampshire.

The animus of the party establishment is even clearer than in the 2016 GOP, with Iowa Democrats’ bolloxing of the vote count and the repeated anti-Bernie bias of MSNBC and CNN.

Sanders looks hard-pressed to grow his core constituency (under-35s), as Trump did his (non-college grads) at this point four years ago — though in time Trump’s ceiling grew higher.

Sanders repels the highest-income Democrats. He got only 16 percent in affluent Bedford and Windham, two of the three New Hampshire towns he lost to Hillary Clinton in 2016. But the vote there — and likely in affluent precincts in future contests — was split between Amy Klobuchar and Buttigieg, each of whom has constituency problems.

Buttigieg gets just about zero support from black voters, who cast more than 20 percent of Dem ballots nationally and a majority in South Carolina. Many Dem voters’ sense of moral superiority derives from the party’s near-unanimous support from blacks as well as the charge, based on thin to zero evidence, of Republicans’ and Trump’s racism. Buttigieg puts that at risk.

Klobuchar has little history with black and Hispanic voters, and her surges in Iowa and New Hampshire were powered by white college graduates, who have been notoriously changeable in their preferences so far. And she doesn’t have nearly the money Buttigieg has raised, much less the billions Mike Bloomberg can spend.

Those problems are nothing compared to those of Elizabeth Warren and Joe Biden, who finished with single digits in New Hampshire and lost two-thirds of their peak support in polls. But why should they drop out before Super Tuesday, March 3, when 39 per­cent of delegate votes will be determined?

One possibly salient difference between the 2016 Republican race and the 2020 Democratic contest so far is turnout. With Trump as a box-office draw, in 2016, GOP primary turnout was up 44 percent from its previous peak nationally.

There was a similar upsurge of Democratic turnout in 2008 but not in 2016 and not so far this year. Dem Iowa turnout was up only 3 percent from 2016 and down 27 percent from 2008. Dem turnout in New Hampshire was up 8 percent from 2016 and 1 percent from 2008.

The 2008 turnout numbers nationally and in the opening states showed the Dems’ huge enthusiasm edge, which was evident in November; 2016 turnout showed Dem enthusiasm flagging and GOP enthusiasm about equal, foreshadowing November.

The 2020 Iowa and New Hampshire turnout is far below widespread expectations. My theory to explain this: Democrats are still in shock over the collapse of the Russia-collusion meme. They fully ­expected to savor Trump being frog-marched out of the White House. And now many of them have less appetite for politics — MSNBC and CNN ratings are down — when it seems he might just get re-elected.

Of course, the 2016 Republican plotline may not be replicated by the 2020 Democrats. Bernie could stumble. Pete or Amy could soar. Bloomberg, unorthodox both in party affiliation and on policy (see his abject apologies for his successful stop-and-frisk policy), could spend his way to the nomination of a party that bemoans the role of money in politics. Maybe it takes one interloper to beat another.