I’ve been critical of Sen. Bernie Sanders Bernie SandersKenosha will be a good bellwether in 2020 Biden's fiscal program: What is the likely market impact? McConnell accuses Democrats of sowing division by 'downplaying progress' on election security MORE and his crew of trolls since the 2016 Democratic primary. Having little concern for the health of the Democratic Party — a political party Sanders (I-Vt.) refuses to join, but wants to be the presidential nominee — Sanders’s backers regularly bash the elected officials I deeply admire who pass critical legislation, win elections and raise funds to keep the party afloat.

Purity tests are their lifeblood, even ones their own candidate of choice wouldn’t pass. Perhaps most concerning, they seek to destroy fellow liberals who deviate from their orthodoxy.

Policy-oriented discussions about the advantages and disadvantages of “Medicare for all” versus ObamaCare are impossible. Don’t think billionaires are all bad? Unacceptable. Think there are some wars worth fighting? You’re a neocon in liberal clothing (clothing that probably costs too much, by the way). Concerned about the feasibility of implementing the Green New Deal? You’re just as bad as the Republicans.

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Their unbridled aggression is well documented. I wrote recently about prominent Sanders surrogate Rep. Rashida Tlaib Rashida Harbi TlaibGeorge Conway: 'Trump is like a practical joke that got out of hand' Pelosi endorses Kennedy in Massachusetts Senate primary challenge The Democratic Party platform represents our big tent MORE (D-Mich.) leading a crowd in booing the mention of Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE in Iowa just a few weeks ago. And on New Hampshire primary night at Sanders’s headquarters, Sanders supporters loudly booed when former South Bend, Ind., mayor Pete Buttigieg Pete ButtigiegBipartisan praise pours in after Ginsburg's death Bogeymen of the far left deserve a place in any Biden administration Overnight Defense: Woodward book causes new firestorm | Book says Trump lashed out at generals, told Woodward about secret weapons system | US withdrawing thousands of troops from Iraq MORE appeared on the televisions. They also chanted “Wall Street Pete.”

Sanders seems to have gotten the message that this type of behavior isn’t going to fly.

In the past few weeks, he has made a concerted effort to stress the importance of coming together to defeat Donald Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE, whoever is the Democratic nominee. This sentiment was front and center in his speech after winning the New Hampshire primary. He told supporters: “What I can tell you with absolute certainty and I know I speak for every one of the democratic candidates is that no matter who wins — and we certainly hope it’s going to be us — we’re going to unite together and defeat the most dangerous president in the modern history of this country.”

That’s the way a guy who has a lot to lose — and a very real chance of getting the nomination — talks. Recent events such as the debacle over the Nevada Culinary Union’s refusal to endorse a candidate and the response of Sanders’s supporters to the union’s criticism of Medicare for all are instructive for the Vermont senator. He cannot afford to have another major organization release a statement like that of the culinary union’s secretary treasurer, who wrote on Twitter: “It’s disappointing Senator Sanders’ supporters have viciously attacked the Culinary Union & working families in NV simply because we provided facts on proposals that might takeaway what we have built over 8 decades.”

What’s more, it hasn’t stopped. The spokeswoman for the culinary union received calls from people telling her that she is a “f---ing whore,” a “bitch” and an “ignorant dumb f--k.” This type of behavior from Sanders’s supporters is not the stuff that wins elections.

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As much as I hate to admit it, Sanders is a legitimate contender this time around. Though he underperformed in Iowa and New Hampshire, where his vote count was the lowest total ever for a winner in the primary, he’s getting considerable buzz and climbing in the national polls. He’s now up over Joe Biden Joe BidenSenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE by more than 4 points in the Real Clear Politics average, a reality that seemed far-fetched just a month ago. He’s also on track to win the Nevada primary next week and is in a strong position to compete in South Carolina.

A new Yahoo News/YouGov poll has more heartening new for Sanders. It finds that he leads all his Democratic rivals in head-to-head contests, including a 4-point lead over Biden, a 15-point edge over former New York City mayor Mike Bloomberg, and 21 points over Sen. Amy Klobuchar Amy KlobucharThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - White House moves closer to Pelosi on virus relief bill EPA delivers win for ethanol industry angered by waivers to refiners It's time for newspapers to stop endorsing presidential candidates MORE (D-Minn.), who is riding high on her New Hampshire “Klomentum.”

Here’s the rub for Sanders. While things are looking up, it’s still early. He’s far from the only act in town and though he’s still the progressive left standard-bearer, voters in New Hampshire and Iowa showed healthy appetites for moderate candidates.

The shift away from Biden as frontrunner surely makes Sanders’s path clearer, but the likelihood of a contested convention grows by the day and he must stay on message. Critically, he’s got to keep his supporters on message. In a tight race, Sanders’s supporters need to get with the program. If they don’t, it may be time for some serious tough love from the candidate himself.

Jessica Tarlov is head of research at Bustle Digital Group and a Fox News contributor. She earned her Ph.D. at the London School of Economics in political science. Follow her on Twitter @JessicaTarlov.