Last week, Israel once again faced a barrage of rockets from Gaza while most of the mainstream Democratic candidates whistled and looked away.

Candidates don’t have to comment on every event happening around the world, but the ­silence of much of the Democratic field feels like more than passing disinterest. And when Vice President Joe Biden offered even a bland message of support for Israel that would have been unremarkable a few years ago, he got massive pushback from the left on Twitter.

And it’s those activists on Twitter, and in the campus fever swamps, who shape the progressive conversation on Israel.

It’s terrifying.

In Thursday’s New York Times, Blake Flayton, a self-described “gay abortion-rights advocate and environmentalist,” wrote of the torrent of abuse he gets at his university because he is a “Zionist” — and how campus rallies about fair wages for custodial staff turn into Israel-hate fests.

He confessed that progressives like him are afraid to speak up about it: “We often refrain from calling out anti-Semitism on our side for fear of our political bona fides being questioned or, worse, losing friends or being smeared as the things we most revile: racist, white supremacist, colonialist and so on.”

The Democratic 2020 field is a lefty college campus writ large. Even when Democrats try to confront anti-Semitism, their message gets muddled — ­because they have to be very careful not to show too much support for Israel.

Consider Bernie Sanders’ essay last week on “How to Fight Anti-Semitism” at the Jewish Currents website. It offered no instruction on fighting anti-Semitism, but instead alternated between criticizing Israel and pretending anti-Semitism doesn’t exist on the left.

Sanders limited all discussion of anti-Semitism to fringe white nationalists. He even blamed regular attacks on Jews in Brooklyn on the ideology of a “whites-only America.” In reality, as Armin Rosen reported in an exposé for Tablet, “the perpetrators who have been recorded on CCTV cameras are overwhelmingly black and Hispanic.”

We should all rail against white supremacists. But Sanders doesn’t dare diagnose, much less try to cure, the disease of anti-Semitism spreading among his hard-left comrades.

Sanders also lent credence to the Palestinian narrative about Israel’s supposedly sinful birth. “Acknowledging these realities,” he lectured, “does not ‘delegitimize’ Israel any more than acknowledging the sober facts of America’s own Founding delegitimizes the United States.”

Wrong. America doesn’t have to defend its very existence. More to the point, Palestinians could have had their own state side-by-side with Israel from day one. They chose to spurn peace, then waged decades of relentless, eliminationist war.

Then again, the wing of the American left that Sanders represents has absorbed that eliminationist mentality. “Palestine will be free, from the river to the sea” isn’t just something Hamas ­fanatics scream; it was chanted at the Democratic Socialists of America’s national convention at the University of Illinois in 2017.

The river is Jordan, the sea is the Mediterranean. If Palestine were “free from the river to the sea,” then there would be no ­Israel.

Mainstream Democrats rolled over for the far left so quickly on Israel that it’s hard to imagine them returning to a sane place. Jews, who vote for Democrats in overwhelming numbers, need to finally wake up to the reality that their party despises the world’s sole Jewish state.

Criticizing any country or its leadership should always be fair game, to be sure, even when it’s our ally. But what is disturbing about such episodes — and the poll numbers they mirror — was summed up in a recent New York Times sub-headline: “President Trump’s hawkish support of Israel has led many Democrats to question the United States’ relationship with one of its closest allies.”

If Trump is for something, in other words, then Democrats have to be against it. That’s absurd and childish. But it’s also based on a lie.

It’s a lie to say Democrats and mainstream liberals are now turning against Israel. That turn happened under the Obama ­administration. Now mainstream Democrats are struggling just to beat back the tide of overt Jew-hatred in their midst.

And they’re doing a feeble job of it. Either the Democratic candidates are too afraid of the hard-leftist base to stick up for Jews and their national homeland — or they don’t want to. ­Either way, this isn’t leadership.

Twitter: @Karol