Vice President JD Vance told European leaders on Friday that their biggest security threat was not military aggression from Russia or China, or election meddling from Moscow. Rather, he said, it was what he called “the enemy within” — their own suppression of abortion protests and other forms of free speech and the sidelining of parties considered extremist.

The address stunned and silenced hundreds of attendees at the Munich Security Conference, a forum where top-level politicians, diplomats and analysts had gathered expecting to hear the Trump administration’s plans for ending the war in Ukraine and Europe’s defenseagainst a rising Russian threat in the future.

Instead, the vice president offered what may be a preview of a new kind of trans-Atlantic relationship under Mr. Trump — one not built on postwar bonds of stability between allied governments, but rather on ties betweenonce-fringe political parties that share a common approach to migration, identity and internet speech.

Mr. Vance singled out his German hosts, who will elect a new chancellor next weekend, and told them to drop their objections to working with a party that has often reveled in banned Nazi slogans and has been shunned from government as a result.

It was an extraordinary intervention in the domestic politics of a democratic American ally, and it brought some gasps in the hall.

He did not mention the party, the Alternative for Germany or AfD, by name but made a direct reference to the longstanding agreement by mainstream German politicians to freeze out the group, parts of which have been formally classified as extremist by German intelligence.