The AfD’s appeal to Vance, Musk and others in President Trump’s orbit is its shared distrust of E.U. regulations, limits on speech as they apply to the far right, and, perhaps above all, the AfD’s signature opposition to immigration.

The leading candidate, Friedrich Merz, a conservative who has adopted many of the AfD’s hard-line positions on immigration, used his closing statement of the debate to promise his voters that he would never allow the AfD into his government. Under him, he said, the firewall would hold.