Jewish conservatives get asked this question more than any other: “Why are Jews liberals?” The question eventually got so tiresome that my father, himself a prominent Jewish conservative, wrote an ­entire book about tracing the history back to Biblical times. You can still buy it on Amazon. So I’m not going to answer it here.

What we know is this basic fact: In national elections, Jews vote for Democratic candidates by a margin of 3 to 1. That number has been fairly consistent through four elections now. It suggests Democrats should have no concerns about keeping Jews in their coalition for another generation.

And yet they do have such concerns. And they should.

This week, prominent Democrats announced a new group called Democratic Majority for Israel, led by the pollster Mark Mellman. He told The New York Times: “Most Democrats are strongly pro-Israel, and we want to keep it that way. There are a few discordant voices, but we want to make sure that what’s a very small problem doesn’t metastasize into a bigger problem.”

The “very small problem” Mellman has in mind is a trio of newly elected Democrats: Alexandria Ocasio-Cortez, Michigan’s Rashida Tlaib and Minnesota’s Ilhan Omar. They seem to have very few foreign policy views aside from a caricature of Israel as an occupying colonial force that sits up at night thinking of new ways to torment Palestinians.

Such ideas haven’t arisen from nowhere. They are the full flowering of decades of leftist propaganda and fashionable campus blatherskite. From such repellent acorns mighty trees grow, as we have seen in Europe. Britain’s Labour party did little to head off the virulent Israel hatred in its ranks, and it is now headed by an out-and-out anti-Semite.

In Britain, once-overwhelming Jewish support for Labour has cratered. A poll before the 2017 election found that only 13 percent of Jews supported Jeremy Corbyn’s Labour because of its horrid record on ­anti-Semitism.

That is why Mellman and his fellow Democrats are smart to be doing this now, before the conflict actually begins to bite. The problem is “very small” at this moment, but the party’s trend line to the left suggests it will grow in force absent some major intervention or ideological change of heart.

Nor are the views of the new, leftist members of Congress completely alien to the kinds of Democrats who take official roles in the party. At the 2012 Democratic National Convention, delegates removed language supporting Jerusalem as Israel’s capital.

When the Obama White House, fearful of losing campaign dollars, intervened to have the language restored, there was a vocal fight on the convention floor. It sounded very much like those who didn’t want the pro-Jerusalem language restored won a voice vote — and when the chair announced otherwise, the hall erupted in boos.

Bernie Sanders came very close to espousing anti-Zionist opinions openly in 2016, and he won 22 states. His path was softened by the hostile posture of President Barack Obama’s administration. Obama claimed to be a friend of Israel, but there was no country or government he criticized more over his eight years — and he concluded his term allowing a UN resolution hostile to the Jewish state to pass without an American veto.

The activist base’s growing antipathy to Israel is less worrisome to friends of the Jewish state than it would have been at any other time in the country’s history, because Israel finds itself in a surprisingly strong position internationally and at home. It has held the line against Palestinian terrorism, and it is working in concord with Arab and Muslim nations in a manner that would have seemed science-fictional at the turn of the century.

What should be concerning is the subject that goes unaddressed in Mellman’s fight: the potential mainstreaming of anti-Semitism in the Democratic Party as represented by the renewed public importance of Louis Farrakhan and the refusal of vanguard figures on the left, like the leaders of the Women’s March, to repudiate his noxious filth.

Here, too, Democrats need not worry today about this electorally or when it comes to votes and donations. Instinctively liberal, Jews are bound to be more alarmed by some of the white-nationalist encroachments into President Trump’s GOP. But the Corbyn example looms large and is arguably far more dangerous to the American Jewish future than anti-Israel sentiment in the Democratic Party is to Israel’s future.