Even so, it is a terrible thing to label candidates as “Russian assets” based on their views alone. It actually helps create its own kind of propaganda victory for foreign rivals, by replacing serious policy debate with rumor-mongering and suggestions of disloyalty.

Ms. Gabbard is an antiwar candidate who holds unorthodox positions on the Syrian conflict. Yet “unorthodox” also describes Senator Bernie Sanders on banking policy, Senator Elizabeth Warren on antitrust policy and former Representative Beto O’Rourke on the free exercise of religion. Challenging orthodoxies is a legitimate and necessary feature of political debate, and if Ms. Gabbard is wrong, her critics should focus on explaining why, not on casting unsupported aspersions on her motivations.

At the core of American democracy, when it works, is the tolerance of a broad range of political viewpoints. The idea is that received wisdom should be tested and challenged, and that doing so makes the United States and other democracies more resilient than their authoritarian rivals. It is also, at any rate, the theory of the First Amendment.