“We’ve learned from labor the meaning of power,” Martin Luther King Jr told dockworkers in 1967. Relearning this today could mean the difference between victory and defeat for the Bernie Sanders movement.

Unions will also be crucial in the general election, particularly in battleground states like Wisconsin, Michigan, and Pennsylvania that flipped to Trump in 2016. And if Sanders does win the White House, implementing his ambitious campaign promises will likely hinge on the emergence of a disruptive labor movement like the one that ushered in the 1930s New Deal.

This should be a cause for concern for everyone involved in Bernie Sanders’s political revolution. With Sanders and his establishment rivals neck and neck in the polls, labor unions — most of which have not yet issued any 2020 endorsements — could determine the Democratic primary.

Bernie Sanders is taking on some of the most powerful economic and political elites in the world, who will do everything possible to stop him. It’s an incredibly tough task. Winning will require that he have working people’s strongest weapon on his side: unions. Without a powerful and combative labor movement, even the most progressive politician cannot get very far.

Labor at the Ballot Box

Though organized labor represents a declining percentage of American workers, it remains one of the largest and most powerful social forces in the country. With more than 14.7 million members nationwide, unions still have more than enough financial and organizational resources to decisively turn electoral tides.

Look at Kentucky, where a teacher-led political insurgency took down Republican governor Matt Bevin this November. Mobilized by the spring 2018 walkouts to save their pensions and fund public education, and outraged by Bevin’s insults against them , thousands of unionized educators formed the backbone of the “Won’t Be Bullied by Bevin” election crusade. In the face of this grassroots upsurge — and buoyed by $1.2 million in teacher union donations — even Trump’s last-minute campaign rally was not enough to save Bevin, who lost by five thousand votes to a Democrat, Andy Beshear, who ran on a strong pro–public education platform.

On election night, Beshear acknowledged the decisive role of unionized school workers: “To our educators, your courage to stand up and fight up against all the bullying and name calling helped galvanize our entire state.”

Organized labor could also provide the margin of victory this primary season. Since the Democratic presidential field is so tightly contested, a decisive intervention by a number of big international unions could swing the election. The vast majority of unions, however, have so far remained reluctant to pledge their support to any candidate. For Sean McGarvey, president of North America’s Building Trades Unions, “We don’t see ourselves making any endorsements any time early . . . if at all.”

This is perhaps a certain type of progress. In 2015–16, most union officials delivered early endorsements to Hillary Clinton, without allowing for any substantive membership discussion or decision-making. Rank-and-filers across the country were incensed by this breach of union democracy, particularly since, for the first time in generations, there was a viable alternative to supporting yet another corporate Democrat.

Union leaderships this time around are trying to wait as long as possible before issuing an endorsement. Avoiding more rank-and-file backlash is only one reason for their reticence. Despite the meager results of decades of dependence on a party funded and controlled by corporate interests, labor’s top brass generally remain tied to the Democratic establishment. Since the party’s kingmakers have yet to coalesce around a candidate, the safest bet in their minds is to wait and see until the primary becomes less fluid.

Even many progressive labor leaders are reluctant to take a stand. In off-the-record interviews conducted for this article, some union officials expressed to me that they’re torn between Bernie Sanders and Elizabeth Warren. Others said they personally support Sanders but are hesitant to open up a potentially divisive endorsement process since their members support various candidates. And, as is true with many rank-and-filers, sympathy for Sanders’s platform is often mixed with apprehension about his perceived electability (despite the fact that recent national polls show him beating Trump by twelve points).

To help overcome such hesitations, Sanders has actively courted unions, contrasted his pro-labor record with the inconsistency of his Democratic rivals, and issued a bold Workplace Democracy plan for doubling union membership. At the recent Teamsters forum in Cedar Rapids, Iowa, Sanders argued that rather than trying to balance between corporate and working-class interests like traditional Democrats, his administration would be a “workers’ government.”

And in a pointed reference to front-runner Joe Biden, Sanders added: “Some colleagues talk about being pro-worker. That’s good—but they voted for some of the worst trade agreements in the history of this country. I am proud that not only did I vote against NAFTA, but I led the effort against those disastrous trade agreements.”

The campaign’s new initiative Union Members for Bernie is bringing together rank-and-file workers to spread the political revolution to their coworkers. And activists within Labor for Bernie, an independent network of union members, are also independently organizing to get their unions to endorse Sanders.

I asked Russ Weiss-Irwin, a Boston middle-school English teacher and Labor for Bernie organizer, about how he responds to coworkers who fear Bernie is “too radical” to defeat Trump:

Even though the media either ignores or attacks him, Bernie is still the most popular politician in the United States, and he inspires more volunteering and donations than any other candidate. So I think Bernie will win because he makes people want to vote for him, not just against his opponents — he makes people believe that the world we deserve is possible.

For anybody who needs a reminder, Hillary Clinton’s 2016 defeat proves that endorsing a Democratic centrist is not necessarily a recipe for electoral success or high union-member turnout. With working-class families stretched thin and searching for anti-establishment alternatives, exit polls showed that 43 percent of union households voted for Trump, the highest Republican result in decades.

If this year’s initial primary endorsements are any indication, a growing number of union activists have learned the lessons of 2016. Sanders is leading the pack when it comes to labor support, having received the endorsements of ten unions, including United Teachers Los Angeles (UTLA), National Nurses United (NNU), and United Electrical (UE). For UTLA secretary Arlene Inouye, “we saw that Bernie Sanders was aligned with our values, our policies, and our commitment to dramatically transform this nation. So it was critical that our union stick its neck out, that we take a bold stand.”

Joe Biden, in contrast, has received only two union endorsements, Elizabeth Warren only one, and Pete Buttigieg zero. This dynamic, however, could change rapidly if leaders of the largest unions decide to throw their weight behind an establishment candidate. Short of winning democratic endorsement processes in these unions, the best case alternative scenario for Sanders’s rank-and-file supporters may be for labor officials to continue sitting out the primary.