Le Pen is now a serious candidate for the French presidency in 2017. Her strong first-round performance in regional elections was not matched in the second round. She faded. But as with Trump, she answers the popular call for an end to business as usual after two Paris massacres this year in which the Islamic State had a role. The three jihadists who killed 90 Friday-night revelers in the Bataclan club were French citizens believed to have been trained in Syria.

“Islamist fundamentalism must be annihilated,” Le Pen says. People roar. “France must ban Islamist organizations,” she says. People roar. It must “expel foreigners who preach hatred in our country as well as illegal migrants who have nothing to do here.” People roar.

There is no question Le Pen is being taken seriously in France. Europe’s watchword is vigilance. Its entire postwar reconstruction has been premised on the conviction that peace, integration, economic union and the welfare state were the best insurance against the return to power of the fascist right.

That conviction is shaken. The rise of the Islamic State, and the Western inability to contain it, leads straight to the Islamophobia in which Trump and Le Pen traffic with success. It would be hard to imagine an atmosphere better suited to the politics of fear. Americans say they are more fearful of terrorism than at any time since 9/11.

“Every time things get worse, I do better,” Trump says. He does. They may get still worse.

The Europeanization of American politics is also the Europeanization of American political risk. The unthinkable has happened in Europe. It is not impossible in America.

It would be wrong not to take Trump very seriously. It would be irresponsible. It would be to forget European history, from whose fascist example he borrows. In Weimar America politics are not what they were. The establishment looks tired. The establishment has not understood the fact-lite theater of the contemporary world.

The Weimar Republic ended with a clown’s ascent to power, a high-energy buffoon who shouted loudest, a bully from the beer halls, a racist and a bigot. He was an outsider given to theatrics and pageantry. He seduced the nation of Beethoven. He took the world down with him.