EDITOR’S NOTE: Each week we cross-post an excerpt from Katrina vanden Heuvel’s column at the WashingtonPost.com. Read the full text of Katrina’s column here.

One moment dominated coverage following the seventh Democratic Party presidential debate last week: a rejected handshake between Senators Bernie Sanders (I-VT) and Elizabeth Warren (D-MA). 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 grassroots 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.

Read the full text of Katrina’s column here.