WASHINGTON — Bernie Sanders promised to deliver a movement. It hasn’t materialized.

The self-described socialist’s plans to overthrow the system, combined with his front-runner status, rattled his own party who quickly assembled last week to bolster former veep Joe Biden.

But it turns out voters didn’t really want what he was offering.

Black voters in southern states and rural white voters in the Midwest shunned Sanders’ vision and turned out overwhelmingly for Biden in the last two contests — leaving the future of Sanders’ second White House bid on the ropes.

States which he won in 2016, including Michigan and Idaho, flipped to Biden and he now enters a new round of important states, including Florida, Pennsylvania and Georgia, where his rival is favored.

Political experts told The Post that Democrats in 2020 are most concerned with defeating President Trump — and believe Biden is the best person to do it.

“I think we now know that Sanders’ showing in 2016 was resistance to Hillary Clinton,” said veteran Democratic strategist Bob Shrum, who ran Al Gore’s and John Kerry’s White House bids.

“The Democratic Party is not a far-left party. It is a center-left party,” he added.

Shrum said Pete Buttigieg’s great finishes in the early states of Iowa and New Hampshire distracted people from the fact that voters were backing a mainstream Democrat and that Sanders was winning by razor thin margins.

“We weren’t paying attention to all the signs of weakness Sanders had. Sanders did well in Nevada because of Latino voters but that was the end of his run,” he said, referring to the senator’s thumping victory in the state’s February 22 caucuses.

The Vermont senator, 78, also banked on the wrong voters, said Jacob Neiheisel, an associate professor of political science at the University of Buffalo.

“Bernie expected youth turnout to increase to levels that we haven’t seen before and pinning his hopes to that was likely a mistake and it’s historically been a mistake,” he said.

“Any time a political movement or candidate relies on either new turnout from non-voters or franchise extension of a massive nature to bring supporters to their cause, historically that has been a disappointment for them.”

“I’m not terribly surprised that the strategy didn’t work.”

Biden’s meteoric rise from zombie campaign to presumptive nominee has been dizzying, with Mike Bloomberg’s late campaign just weeks ago calling for him to drop out or risk handing the nomination to Sanders.

But Biden was always the front-runner, the experts said. While the former veep is a gaffe-prone candidate who is weak on the campaign trail, that doesn’t change how people vote.

Biden is also far more palatable than Sanders and his surrogates like firebrand Rep. Alexandria Ocasio-Cortez who have shown absolutism on the trail — telling voters that if they weren’t with his movement, then they were against it.

“They like Biden. It’s hard not to like Biden,” said Shrum, who has known the former Delaware senator for decades.

Neiheisel also pointed to Sanders’ likability problem with voters who don’t identify with his young and progressive base.

“Middle of the road voters in suburbs who maybe are on the fence between Republican or Democrat are moving into the Democrat fold,” Neiheisel said.

“There are folks who don’t like President Trump but have enough wealth accumulated that they don’t like Bernie either, so they’ve gravitated toward the Biden camp.”

After a string of victories in the important Super Tuesday races, Biden marches toward the Democratic National Convention in July with a delegate lead — 857 compared to Sanders’ 709.

The road ahead will be tough for Sanders, who is expected to lose the delegate-rich states of Florida, Illinois and New York after already conceding big prizes such as Texas and Michigan to his rival.

After being written off, Biden now seems on his way to a general election showdown with Trump in November.

“Its unprecedented, the last 10 days of warp speed change in the Democratic Party,” said Shrum.

“Nobody has ever been shellacked in the early contest the way Biden was and come back and won the nomination,” he added.