More than three million people marched through big cities and small towns across the nation the day after Donald Trump’s inauguration. These actions were unprecedented; they were the largest public actions against a sitting president in the history of the United States, and came on his first full day as president.

The actions, led by women, were not organized by mainstream political organizations and featured a patchwork of demands ranging from expansion of reproductive rights and an end to deportations. The three million participants included people from all over the political spectrum — from diehard Democratic Party loyalists, to anarchists and socialists. But most importantly, they included people who had never been politically activated.

These actions might be puzzling to some, even for people who are sympathetic to a show of force against the most unpopular recently elected American president. Why would you protest an elected official who has not even had the chance to do anything bad?

To explain why, it’s useful to look at the strategy behind organizing a union contract campaign.

When negotiating a union contract, the most important piece for the workers is the strategy to build power at the table. Management’s goal is to push through a contract that will cost the least amount of money and take away as much autonomy from the worker as possible.

The workers’ side wants a contract that will allow them to use their own experience and creativity in the work they do and provide pay and benefits that allow them to lead decent lives. To get there, they need real leverage over management. Having logic or morality on their side is not enough; workers need actual power that they can wield against management to force the boss to do what they want.

One common tactic we often use at the onset of negotiations to build that power is a contract rally. The power we wield is proportional to the size and the composition of the rally.

If the rally only includes members of the bargaining team, the group of members who are handling the nuts and bolts of the contract negotiations, the union is clearly weak. But if the bargaining team is able to mobilize members around it, it shows that the workers have built a strong sense of solidarity with each other. It also raises the possibility that a strike or another kind of workplace action is possible — the last thing the bosses want.

But stopping operations, especially in the public sector, is usually not enough for workers to win. Without the support of a community, workers can be replaced during a strike. Workers therefore need to mobilize the people who are not members of their union but do have power over management.

In a teachers’ union contract campaign, for example, parents and community members need to be part of that rally. When they show up, the specter of the community’s ability to vote out school board members or take their kids out of the schools is raised. In private-sector negotiations, the presence of consumers at the rally raises the possibility of a boycott or a public relations disaster.

Management cares about its bottom line and little else. If workers can put the bottom line at risk, they have leverage at the table.

On Saturday, marchers showed their leverage over the president, essentially declaring themselves ungovernable. Trump clearly laid out an agenda of taking away the rights of every marginalized and exploited group in America while on the campaign trail. This was a preemptive show of force against that agenda.

The power of the march was in its diversity. Had only the “usual suspects” turned out — professional organizers, community leaders, political militants, and politicians — it could have been easily dismissed by Trump. Instead, it was a broad coalition, much like the most successful contract campaign rallies. The marches’ success can be measured in part by how it seemed to send the Trump campaign into a tailspin on its first full day in office.

The women’s march protests worked in the same way that a good union contract campaign works: the country engaged in an unprecedented and preemptive show of force against Trump. The marches’ messaging wasn’t perfect or even fully coherent. But that messaging was less important than the enormous collective muscle-flexing that so many in the country carried out. To defeat Trump’s reactionary agenda, this is exactly the kind of action we will have to engage in.

Turning this incredible moment into a successful movement requires everyone affected by the Trump agenda to show up for each other — and not just online. This means swallowing our sense of pride or purity and working with people whose tactics may seem too milquetoast — or too militant. These won’t be fights where we can afford to be splintered and stay home when our immediate rights aren’t at stake.

Our ability to defeat Trump’s agenda is directly proportional to the power we’re able to amass against him. Saturday’s marches bode well for our ability to build that kind of power under President Trump.