One moment dominated coverage following the seventh Democratic Party presidential debate last week: a rejected handshake between Sen. Bernie Sanders (I-Vt.) and Sen. Elizabeth Warren (D-Mass.). It was 14 seconds in a night that lasted two hours, in a campaign that has been underway for more than a year, but the handshake capped several days of vitriol by the Sanders and Warren campaigns and their supporters. The vitriol has been surprising given the goodwill amassed through the candidates’ previous nonaggression pact. More importantly, it has jeopardized the best chance that progressive Democrats have had in a generation to put a candidate who shares their values in the White House.

Progressive groups recognize the danger of the moment, and have called on the campaigns and their supporters to cool the attacks. Additionally, 18 grass-roots groups have initiated a unity campaign called “Progressives Unite 2020,” affirming that they would work to defeat “candidates supported by the corporate wing, instead of fighting each other.” The Sunrise Movement, which has endorsed Sanders, pointed out that “infighting between Sanders and Warren only benefits big oil, fossil fuel billionaires, the GOP, and the moderate wing of the Democratic Party.” Meanwhile, the Working Families Party, which has endorsed Warren, reminded its members that “Warren and Sanders have much more that unites them.”

While the candidates would be wise to stop the personal attacks, it’s a part of politics for them to draw contrasts. So it’s up to the broader progressive movement to make the comparisons we know exist. It’s up to movement leaders to flip the script. They can do so in a number of ways: by making the case that these two progressive candidates should work together (as Democracy for America did), by waiting to endorse (as many labor unions are doing), by endorsing both Sanders and Warren, or, as the American Prospect’s Harold Meyerson put it, simply by making Sanders and Warren a greater priority than Sanders vs. Warren.

Of course, unity for unity’s sake isn’t enough. Progressive Democrats should unite for the sake of the American people, whose lives would improve as a result of Medicare-for-all, a Green New Deal, a wealth tax, student-debt cancellation and other progressive policies being debated on the national stage.

Progressives who, both before this latest back-and-forth and now, are lifting up both Sanders and Warren, are being principled and strategic. In our winner-take-all political system, the candidate who wins the nomination controls the direction of the party. Multiple progressive front-runners only increase the party’s chances of having a left-of-center nominee and uniting around a left-of-center platform. Establishment Democrats, after all, will be united in their goal to keep the center of the Democratic Party at the core of the Democratic Party. Former secretary of labor Robert Reich remarked recently, “If progressive Democrats don’t hang together, corporate Democrats will hang them separately.”

I warned in October that “some are waging a campaign to drive a wedge between the Sanders and Warren camps that, if successful, could severely deflate the energy among progressives.” Those efforts continue. They are sapping the progressive movement of its focus on the issues that matter to the American people, and dampening the enthusiasm of supporters past, present and potential. That’s no way to expand the progressive lane.

And a fractured movement foreshadows trouble for progressives at the Democratic National Convention in Milwaukee. With the possibility of a brokered convention, progressives would be better off entering with 50 percent or more of the delegates. Reaching that critical threshold between Sanders and Warren would increase the likelihood that a progressive candidate could obtain the nomination on the first ballot, before superdelegates weigh in and establishment forces may be better positioned to seize power.

Now, some fervid supporters would say that the attacks we’ve seen over the past weeks are necessary — that hurling insults about Sanders’s alleged sexism or Warren’s alleged elitism is what the primary is for. But in a primary in which some centrist candidates believe that collecting $2,800 checks from lobbyists is the same as collecting only grass-roots dollars, in which we are weighing the boldness of the candidates’ health-care plans and the levelheadedness with which they’d redefine the United States’ role in the world, there are bigger issues for progressives to tackle than each other. Ultimately, progressives uniting in 2020 will be the only way progressives win in 2020.

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