Minutes before Republicans voted unanimously against impeaching President Trump last month, House Majority Leader Steny Hoyer extolled their predecessors. “The pages of our history are filled with Americans who had the courage to choose country over party or personality,” he reminded them. “I urge my fellow colleagues in the House and, yes, in the Senate, to look into your soul.”

Hoyer’s call went unheeded, of course, but he was right to focus on the courage his colleagues would need rather than trying to persuade them with evidence. It may be too much to hope to change any Republican minds; now we must hope that those who know their party line is untrue will have the courage to speak their minds.

It’s easy to criticize those Republicans for remaining silent. But in fairness, it’s hard to stand up against your own team, particularly when you must do it alone. Just ask Senator Jeff Flake or Congressman Justin Amash, who were viciously attacked by the President and his allies and quickly found themselves homeless in the Republican Party. Worse, the benefits of speaking out may seem minimal: there’s no apparent effect of doing so, other than to be marginalized and vilified by your colleagues and the president.

But this overlooks a key benefit of speaking out publicly, which is to embolden others who are struggling to muster the courage to do so. The current landscape facing a prospective defector is a Republican Party in near-perfect lockstep. But if even a few people spoke out in even a small way, that unanimity could begin to crumble. This matters because, as economist Robert Frank makes clear in his forthcoming book, “Under the Influence,” beliefs are contagious: what we see when we look outward can matter as much or more than what we see when we look inward.

Frank’s argument builds on the idea that although we like to think we develop our opinions independently, in fact we often look to others when deciding what to believe and do. Someone becomes 25 percent more likely to smoke, for example, when the percentage of smokers among her friends rises from 20 to 30; and a new rooftop solar installation in a neighborhood typically leads other neighbors to take the same step. What others do is far more influential than most of us realize.

Consider a classic study by psychologist Solomon Asch in the 1950s, who asked which of three lines in one drawing had the same length as a single line in a second drawing. To any neutral observer, the answer was obvious, and when people answered individually, they were correct more than 99 percent of the time. But when faced with the same question in a group of eight, where the previous seven people had all given the wrong answer (they were Asch’s confederates), people wavered: only 63 percent gave the right answer. Notably, when even one of the seven others dissented, the correct response rate rebounded to more than 95 percent.

Conventional wisdom holds that in the face of compelling evidence assembled in the House, McConnell and Senate Republicans are going to acquit Trump. But before that can happen, the Senate must agree on rules governing their deliberations, and only four Republican senators’ votes are needed to assure a procedurally fair trial. If there’s to be any chance of mustering those votes, it won’t be because Democrats show Republicans evidence they’ve already seen; it will be because those Republicans found the courage to speak their truth.

I recently asked Frank what might make that outcome more likely. His first advice was to begin with small steps. It’s much easier to vote to allow witnesses in a Senate trial, he said, than to vote for the president’s removal. But even this seemingly small step can serve as an important signal of senators’ willingness to speak up. It’s also one that’s relatively easy to defend: let’s get the facts on what actually happened rather than hide our heads in the sand.

Frank also pointed out that even small actions can alter or reinforce your identity in ways that matter. Carrying a reusable bottle has only a negligible effect on the environment, but it can serve as a reminder that the environment matters, making you more likely to support candidates who will protect it. Similarly, insisting on a fair trial could serve as a reminder that your first allegiance is to the Constitution, not an individual or party.

Next, Frank suggested that there’s safety in numbers. It’s easy to isolate an individual from the pack, as when former Republican Justin Amash was forced out of the party when he publicly announced his support for impeachment. But preserving the apparent unanimity in the GOP would become virtually impossible if the number of dissenters grew even slightly.

Finally, Frank pointed to the work of economist Timur Kuran, whose book “Private Truths, Public Lies” noted that many political revolutions (like the fall of Communism and the French Revolution) caught the world almost completely by surprise. This happened because people’s private views differed from those that were prudent to express publicly. But once even minor expressions of dissent began to surface, they quickly snowballed. And with each additional citizen who spoke out against an oppressive regime, it became safer for others to do so.

Circumstances confronting today’s Republicans are similar: Although former Arizona Senator Jeff Flake reports that at least 35 of his former Republican colleagues would vote to convict Trump if they could do so in private, pundits insist that none of them would be willing to cast such a vote publicly. Maybe so, but an inherent feature of contagious processes is that dramatic change is sometimes far more likely than it appears. In any event, one thing is clear: If we do see a dramatic shift among Republican senators, it most likely will have begun with only a small handful of them having the courage to stand up to Mitch McConnell and support a majority vote for a procedurally fair Senate trial.