This post is part of Polyarchy , an independent blog produced by the political reform program at New America , a Washington think tank devoted to developing new ideas and new voices.

Tuesday’s election could pose an unprecedented challenge to American democracy. Donald J. Trump and many of his supporters may not accept the results. But this year’s presidential campaign has already entered uncharted territory. Unlike any other major party candidate in modern US history, Trump has openly and repeatedly attacked basic norms of our democracy — threatening to jail his opponent, calling on a hostile foreign power to hack her campaign, and questioning the legitimacy of the election on the demonstrably false grounds that it is “rigged.“

Republican Party leaders have seemed at a loss for how to respond to their candidate’s anti-democratic behavior. Focused on winning the election or avoiding the wrath of right-wing allies and the base, most GOP politicians have either grudgingly endorsed Trump or adopted a strategy of silence or ambiguity. Few prominent Republicans with a political future have fully broken with him.

If it continues through November 8, the Republicans’ strategy of equivocation could prove to be a tragic mistake. Should Trump question the legitimacy of the election’s outcome or condone extremism or violence in its aftermath, Republican leaders must set aside normal politics and join Democrats in finally drawing the line against such behavior — even at the risk of angering the party base.

Why is the Republican response so critical? The great political scientist Juan Linz devoted much of his career to understanding why and how established democracies die. Having spent years researching the reasons for the tragic collapse of democracy in 1930s Europe, Linz proposed a “litmus test,” a list of actions by politicians that can put democracy at risk. These warning signs include a refusal to unambiguously disavow violence, a readiness to curtail rivals’ civil liberties, and the denial of the legitimacy of an elected government.

Donald Trump has clearly passed Linz’s anti-democratic litmus test. He has encouraged violence among supporters (offering to pay their legal fees), pledged to jail Hillary Clinton and take legal action against unfriendly media, and suggested that he might not accept the election results. Such acts are unprecedented among major American candidates, but they are precisely the kind of behavior that Linz and other scholars have identified as preceding democratic breakdown in interwar Europe.

But the problem is not just Trump. Linz’s research shows that what undermines democracies in crisis is not only the behavior of extremists but also that of mainstream politicians who — out of fear, ignorance, or political calculation — tolerate and even facilitate their ascent.

In Italy, when Mussolini and his National Fascist Party first arrived on the political scene, Liberal Prime Minister Giovanni Giolitti helped to “normalize” them and bring them into mainstream politics by forming an electoral alliance — not because he was pro-fascist but because they could help him win the coming election. And in Germany, conservative politicians formed a “fateful alliance” with Hitler, catapulting him into national prominence out of not conviction but expediency; they hoped to tap into the support of his voters while steering him toward “responsible” positions. In 1933, the conservative President Paul von Hindenburg named Hitler chancellor, though he loathed him, because Hindenburg believed it was the only way to preserve a right-wing government.

Our point is not that Trump is a Hitler-like figure or that the United States is on the brink of fascism. Rather, the experiences of interwar Europe remind us that Western democracies are not immune to collapse. We would be foolish, therefore, to ignore the lessons they provide.

What we can learn from 1930s Europe is that democracy is imperiled when mainstream politicians do not act decisively against anti-democratic figures, when they continue to let day-to-day political calculations guide them, when they align with extremists because it is politically useful, or when they simply tolerate them via silence or ambiguity because they are popular with “the base” and it is politically costly to confront them. As Linz wrote, the tragic demise of many democracies can be traced to a mainstream party’s “greater affinity for extremists on its side of the political spectrum than for [mainstream] parties close to the opposite side.”

To ensure democracy’s survival, mainstream politicians must vigorously oppose anyone who threatens democratic norms, even if it means temporarily joining forces with their partisan rivals. And even if it means likely political defeat.

The lessons for Republicans today are painful but clear. It is not enough for Republican leaders to dissociate themselves from Trump’s most outlandish comments or simply distance themselves from his candidacy. They must actively oppose him, because he is an anti-democratic figure who threatens our country’s institutions. Republicans must build a wall — not of brick and mortar, but of democratic ideals — to defend our democratic norms, isolating those who violate them.

The Republican response to Tuesday’s election will be a critical test — with potentially momentous consequences. If Trump or any of his supporters refuse to accept the results or threaten or condone violence, Republican leaders must rise to the occasion, joining forces with Democrats in unambiguously denouncing such behavior. They must sever all ties with anyone who engages in extremist or anti-democratic rhetoric. Such a response may infuriate some hardcore supporters, but it is essential to preserving our democratic system.

Winston Churchill once declared that political leaders “must be judged in the testing moments of their lives. Courage is rightly esteemed the first of human qualities, because ... it is the quality which guarantees all others.”

This is a Churchillian moment for the Republican Party. If Republican leaders find the political courage to stand up to a popular demagogue and his movement, they will help avert a dangerous spiral that could imperil our democracy — and they might even find a new basis for a re-founded Republican Party. If they do not, history will not be kind to them.

Steven Levitsky and Daniel Ziblatt are professors of government at Harvard University.