Froma Harrop is a syndicated columnist. Follow her on Twitter: @FromaHarrop The opinions expressed in this commentary are hers; view more opinion articles at CNN.

(CNN) The recent primary losses are not the first time the air went out of the movement-left's visions of inevitability. It also happened during the 2016 presidential primaries when -- for all of Bernie Sanders' massive rallies and coverage by an obsessive media -- the independent from Vermont lost to Hillary Clinton. And he lost by a not insignificant margin of nearly 3.7 million votes.

The problem with left-wing movements isn't so much "the left-wing" as it is the "movements." Movements sometimes have a totalitarian edge that cannot abide dissent, or even a hint of it. They're "us" versus "them." In the case of left-wing movements, the "them" is often fellow Democrats.

You saw that way back in 1972, when Sanders was running for Vermont governor. As Mother Jones reported, a fellow Vermont activist named Greg Guma asked Sanders the innocent question of why he should vote for him.

Sanders' response: "If you didn't come to work for the movement, you came for the wrong reasons. I don't care who you are; I don't need you."

The painful reality for the Democratic Party is that much of the movement left doesn't necessarily care whether Democrats win. Recall the election of 2000, when Green Party candidate Ralph Nader siphoned off enough liberal votes to deliver victory to Republican George W. Bush.

Democrat Al Gore and Bush had been running neck and neck. Despite reports that Republican groups were running ads for Nader, significant voices on the left were urging liberals to vote for Nader.

Why? Here is how Barbara Ehrenreich put it in The Nation: "What I fear most about a Gore victory — yes, I said victory — is its almost certainly debilitating effect on progressives and their organizations."

She wasn't alone. Celebrity agitator Michael Moore was bouncing on and off stages in liberal cities urging audiences to vote for Nader.

"A vote for Gore is a vote for Bush," he bellowed. "If they both believe in the same thing, wouldn't you want the original rather than the copy?"

Shortly after Donald Trump won in 2016, I attended Moore's one-man Broadway show. He opened with "How the f—- did this happen? The second time in the last 16 years we got the most votes!" I nearly choked.

Fast forward to 2018. Alexandria Ocasio-Cortez, a "Democratic Socialist," set off a political earthquake when she dethroned a powerful Democrat in a congressional primary. The district covers parts of the Bronx and Queens.

Ocasio-Cortez became an instant media and left-wing sensation. Articulate and charming, she became the subject of obsessive speculation. But thrust onto major talk shows, she revealed some serious holes in her understanding of national issues.

Whatever, Ocasio won fair and square, defeating an incumbent who apparently forgot to campaign. An ardent Sanders follower, she then retreated into the movement mentality.

The defeated Joe Crowley, who immediately conceded and promised to support her, came under attack for not doing something he couldn't do without taking extreme steps -- remove his name from the ballot on a small-party line. "Alexandria," Crowley tweeted , "the race is over and Democrats need to come together. I've made my support for you clear and the fact that I'm not running."

But Ocasio-Cortez took the movement path, putting her energies into helping defeat, not Republicans in November, but "establishment" Democrats in the primaries. So she and Sanders jetted around the country, endorsing left-wingers sure to lose in a general election.

Democratic primary voters generally ignored them. In the Democratic primary for governor of Michigan, Abdul El-Sayed lost to the establishment choice, former state Sen. Gretchen Whitmer, by more than 20 percentage points. In Kansas, a former Sanders campaign aide, Brent Welder, lost to lawyer Sharice Davids in his bid to oppose Republican Rep. Kevin Yoder in November.

Some items on the far-left agenda -- closing down the Immigration and Customs Enforcement agency, for example -- turned off many Democrats.

Other voters were undoubtedly hungry for candidates who could win a House seat in a purple or red district.

This is not to dismiss Ocasio-Cortez's considerable talents. She ran a brilliant campaign, both on social media and on the city pavements. And, unlike the movement-left professors dropping pearls of theory on the working masses, she actually comes from "the people." I hope she matures and drops movement politics like training wheels on a bicycle.

Nader played the genuine spoiler in 2000, but Sanders came outrageously close in 2016. Even after Hillary Clinton was strongly favored to become the nominee -- and Donald Trump became her likely opponent -- Sanders turned his biggest guns on Democrats.

During one primary debate, Sanders condemned Clinton in what he posed as a question: "Are you qualified to be President of the United States when you're raising millions of dollars from Wall Street, whose greed, recklessness and illegal behavior helped to destroy our economy?"

He considered appealing to the party's superdelegates to overturn the voters' choice and make him the nominee.

Then, to obtain concessions, he refused to endorse Clinton almost to the party's convention.

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Trump picked up on Sanders' brutal critique and ran with it during the campaign. And small wonder Russian trolls working to elect Trump fired up tension between the Sanders movement and mainstream Democrats.

As for policy, there's often not a huge gap between far left and centrist Democrats. Most everyone wants Medicare for all. The difference is that the far-left wants to move everyone there tomorrow. The centrists would start by letting those just under the 65 age-of-eligibility buy into Medicare. That was Clinton's stance.

The gradual approach is probably the more effective way to get there because it doesn't scare the public with an overnight remake of health coverage. But Sanders insists on inserting the more controversial term " single payer " into Medicare for all.

At some point, so-called establishment Democrats should stop trying to win the far-left's love -- unwinnable because they are not part of the movement -- and tell them this: If you didn't come to ultimately work for the party, you entered the party's primaries for the wrong reason.