The Democratic Party could soon be taken over by a leftist who has never formally been a member (even if he signed a pledge to govern as a Democrat should he make it to the White House).

If it’s any consolation to Democrats, it’s a version of the same wrenching dislocation that has beset the center-left throughout the Western world.

One reason Bernie Sanders winning the Democratic nomination is entirely imaginable is that it wouldn’t be a freakish occurrence outside the experience of other advanced democracies. In fact, it would be entirely consistent with the travails of traditional center-left parties from France to Sweden.

Seen from a broader perspective, Sanders doesn’t represent a revolution so much as a lagging indicator — indeed, the British Labour party has already been there, done that.

Across the European landscape, center-left parties have foundered in recent years, often torn apart by crosscurrents over immigration and by tensions between socially progressive urbanites and traditionalist working-class voters.

“As social-democratic parties declined due to the weight of structural changes,” Yascha Mounk wrote in an essay last year in Democracy: A Journal of Ideas, “a new crop of left-wing politicians argued that their troubles were owed to the ­embrace of a more moderate (or ‘neoliberal’) set of policies.”

What was necessary, in this view, was “an unabashed emphasis on the economic interests of the working class, coupled with the full-throated promise of social revolution.”

Sound familiar? Sanders bears the closest resemblance to his equally aged and disheveled ideological cousin from the United Kingdom — Labour leader Jeremy Corbyn.

Corbyn crashed the gates of the Labour party as, essentially, an outsider. He rose on the strength of a left-wing grassroots movement and won ­Labour’s leadership contest in 2015 thanks to newcomers who could vote in such a contest for the first time. An unlikely icon for younger voters, he drew enormous crowds and, unavoidably himself, had a distinctive charm for his supporters.

He also, like Sanders, had a history of sympathy for left-wing thugs, a bitter hostility to Western power, a motley collection of kooky allies and an utterly fantastical domestic program.

After losing two elections, Corbyn is back in the dustbin of history, while Comrade Sanders may be tracing his ascent on a larger scale.

A major commonality with the European experience is young voters who are disenchanted with the institutions of Western capitalism, worried about affording a house and paying off student debt. They have no experience with or ­attachment to old party loyalties and are drawn to exhilaratingly radical and simple ideas.

If Sanders does indeed win the nomination, these kids who aren’t all right will be the shock troops of his revolution. In New Hampshire, Sanders crushed among young voters, winning an outlandish 51 percent of voters ages 18 to 29 and a solid 36 percent of voters ages 30 to 44. (If there were no Democrats over the age of 45, Sanders would sweep all before him.)

These kinds of voters are much more bonded to Sanders than to the Democratic Party — a poll a couple of weeks ago found that only 53 percent of Sanders supporters said they’d definitely back the Democratic nominee if it’s not Bernie.

The two political parties are such towering edifices of American civic life that they aren’t ­going to dwindle away to nothing in one election like, say, France’s Socialists did a couple of years ago. Instead, the sign of their relative decline is that they are vulnerable to takeover by political entrepreneurs bringing along their own personal political movements.

This obviously happened to ­Republicans with the rise of ­Donald Trump in 2016. Tellingly, the main vehicle opposing Sanders in the Democratic nomination battle in 2020 could end up being Mike Bloomberg, another (until ­recently) non-Democrat with the support of his own de facto private political party via his prodigious bank account.

That Bloomberg may prove the only way for the US center-left to avoid the fate of its international counterparts is a sign of its desperation — not its vitality.

Rich Lowry is the author of the new book “The Case for Nationalism: How It Made Us Powerful, United and Free.”

Twitter: @RichLowry