The Democratic warhorse Dianne Feinstein is warning that Bernie Sanders’ campaign against Hillary Clinton could turn the party’s convention in Philadelphia into the kind of disaster that erupted in Chicago in 1968.

“It worries me a great deal,” Senator Feinstein tells CNN.

I can understand why. I was there. Covering the Democrats’ 1968 convention for one of America’s greatest newspapers, the Anniston (Alabama) Star, I saw a bitterly divided party, riven by the Vietnam War and Jim Crow, met with violence in the streets.

The convention finally handed up, in Vice President Hubert Humphrey, one of the finest liberals in history. The Democrats failed to heal their rifts, though, and lost to Richard Nixon. The party would never be the same.

There are no perfect parallels between the Democratic Party today and the one that came a cropper in 1968. But the mob mentality of some of Sen. Sanders’ thugs, on display in Nevada, must make Feinstein wonder.

Like Mrs. Clinton, Humphrey was an establishment figure challenged by a radical left-winger. Back then it was Sen. Eugene McCarthy, an opponent of the Vietnam War, and a small army of nihilists.

McCarthy’s showing in the Democratic primary in New Hampshire had so shaken President Lyndon Johnson that he announced he wouldn’t stand for another term. Sen. Robert F. Kennedy had already jumped into the race, and would soon be slain by an assassin.

It wasn’t only McCarthy and the protesters who gathered in Grant Park that made for drama in Chicago. That was also the year in which the Democrats purged their segregationist heritage, personified by Alabama’s ex-governor, George Wallace.

Ostensibly a Democrat, he was running for president as an independent. A three-way fight erupted over who was going to get the convention votes for Alabama. Wallace’s forces were, in a hearing I covered, soundly defeated.

Power shifted to a lawyer from Birmingham, Robert Vance, who was loyal to Humphrey, with all his integrationist enthusiasm. Vance was also a Vietnam War hawk, at least in those days.

One night he gave me a lift to the convention center. As we passed Grant Park, where the anti-war protesters were battling Mayor Richard J. Daley’s police, he made a sardonic remark about the tear gas as he suggested we close the window.

Vance didn’t mean to suggest it was a laughing matter. What became known as the Battle of Michigan Avenue involved as many as 10,000 demonstrators and maybe twice as many police and National Guard.

The protesters called it a police riot (a hysterical Abraham Ribicoff, then a senator from Connecticut, likened the police to the Gestapo). The American people sided with Daley and his officers, and handed the presidency to Nixon.

That’s what Mrs. Feinstein fears in Philadelphia. It’s no doubt why Donald Trump is moving so rapidly to try to unify the Republicans (though violence threatened against him is not from within the GOP but, like Chicago, from the radical left).

Humphrey won the nomination not by competing in the primaries but by caucus delegates. The debacle in Chicago led to the creation of a “reform” commission, which upended party rules and set the party on its leftward course.

The commission was headed by a cynical senator named George McGovern, who stepped down from its leadership to run for president in 1972. The big, anti-Communist labor leaders, like George Meany, were locked out.

McGovern went down in one of the most complete defeats in history (he won one state and 17 of 537 electoral votes). Yet even today, poor Hillary Clinton is feeling the consequences — or, in modern lingo, “the Bern.”

That year, George Wallace tried again for president, only to be cut down in an assassination attempt and paralyzed. He eventually became a born-again Christian, repented his segregationist views and won a last term as governor — with the support of black voters.

Vance, the Democratic Party loyalist, became a judge. One day a package arrived in the mail. It exploded when he opened it, making him the only United States circuit judge ever assassinated.

Vance’s killer, a Southern racist, was convicted by a young prosecutor named Louis J. Freeh, who went on to head the FBI. The killer sits on death row, while both political parties scramble to keep America’s political demons at bay.

This column first appeared in the New York Post.