What happened to Joe Biden?

It’s astounding but true: Three of the past four presidents were born within a few months of each other in one year — 1946. Together, Bill Clinton, George W. Bush and Donald Trump embody the outsize influence of Baby Boomers on our politics and culture.

Joe Biden and Bernie Sanders, both members of the Silent Generation, are challenging Boomer dominance this year. But while the Vermont ­socialist amasses vast crowds of young people, has no problem sticking to his message and has bounced back surprisingly from a heart attack, the ex-veep continues to struggle.

Biden’s New Hampshire effort is just sad. Last week, he had to skip a full day of campaigning in New Hampshire to reassure donors and reorder his campaign staff; on Tuesday, he announced he wouldn’t stick around in the Granite State to, er, celebrate the results.

He is low on money, and the crush of ad spending by Michael Bloomberg and Tom Steyer in critical states means that he can barely afford to compete in key markets. His events seem exhausted and strained, his voice quavering between a harsh staccato and a whisper.

Democratic nomination battles tend to favor the new and inspiring instead of the old and tested. Barack Obama, Clinton and JFK all exploited this — and Biden has not been new and inspiring for nearly 30 years.

His age shows in his meandering style and his culturally odd references, such as calling a polite young female college student this week a “lying dog-faced pony soldier.” Some writers compare Biden’s stream-of-unconsciousness reminiscences on good times and bad to “onion belt” stories — a reference to old Grampa Abe Simpson’s meandering, pointless stories, featuring the immortal line: “So I tied an onion to my belt, which was the style at the time.”

But it isn’t just his age and ­incompetence that are killing the Biden campaign; Sanders’ excited crowds illustrate that. It’s that Biden is out of tune with the powerful ideological forces coursing through the Democratic Party: ­socialism and wokeness.

This isn’t about winning the general. Biden, despite his struggles, consistently remains the strongest general-election candidate against President Trump. But the party’s media elite and young base long for qualities he doesn’t offer: part economic radicalism in the Sanders style, part woke-ism. The old white guy who’s been in DC forever offers nothing in this regard. Especially when the alternative is the lure of democratic socialism.

Biden doesn’t call for destroying private health care. But choosing him also runs counter to the whole philosophy of woke progressivism. It would require the base and the party’s blue-check elite to take a step back from 2016, demonstrate humility and recognize that the country isn’t where the progressives want to be yet, culturally or economically.

Such a party would admit that James Carville’s recent critique of Democratic elitism was thoroughly justified, and that voters would rather know their children are going to get a good education than, as Elizabeth Warren recently promised, that the next secretary of education must first pass muster with a randomly ordained trans child. While Carville speaks for most Americans, he may not speak for most Dem activists.

Biden’s firewall was always ­going to be African Americans in the South — generally more pragmatic voters, who aren’t as down with the woke agenda as national progressives are. These voters are essential to Democrats’ national hopes this fall. But pitted against a media shot through with wokeness and progressive activists who reject compromise and half-measures, that cohort now lacks a voice other than Biden.

We find ourselves now in yet ­another presidential cycle in which elite Democrats and their media allies have structurally compelled themselves to pre-empt and discard the voices of Southerners and minorities — two classes of voters they need to win.

In that light, the fall of Biden isn’t really about Biden at all. It is about who actually runs the left-wing coalition. Don’t be surprised if it turns out to be upper-income, woke, white Baby Boomers from the North. In politics as in life, they keep on winning — the other generations be damned.

Ben Domenech is the publisher of The Federalist.