[I]t is her “Declaration of Conscience” speech for which [Margaret Chase Smith] is best remembered. It was 1950 and she was increasingly disturbed by Sen. Joe McCarthy’s anticommunist crusade. In February he’d made his speech in Wheeling, W.Va., charging communists had infiltrated the U.S. government at the highest levels. He claimed to have 205 names of known communists; in later statements he put the number at 57 and 81.

The base of the party found his opposition to the communist swamp in Washington electrifying. His wildness and disrespect for norms was seen as proof of authenticity: He’s one of us and fighting for us.

Smith was anticommunist enough that Nikita Khrushchev later described her as “blinded by savage hatred,” and she was certain communism would ultimately fail. But you don’t defeat it with lies.

She always listened closely when McCarthy spoke. Once he said he was holding in his hand “a “photostatic copy” of the names of communists. She asked to see it. It proved nothing. Her misgiving increased.

She didn’t want to move against him. She was new to the Senate; he was popular in Maine. She waited for her colleagues. They said nothing.

Finally she’d had enough. On June 1, 1950, she became the first Republican to speak out. On the way to the chamber Joe McCarthy suddenly appeared. “Margaret,” he said, “you look very serious. Are you going to make a speech?”

“Yes,” she said, “and you will not like it.”

When history hands you a McCarthy—reckless, heedlessly manipulating his followers—be a Margaret Chase Smith. If your McCarthy is saying a whole national election was rigged, an entire system corrupted, you’d recognize such baseless charges damage democracy itself. You wouldn’t let election officials be smeared. You’d stand against a growing hysteria in the base.

You’d likely pay some price. But years later you’d still be admired for who you were when it counted so much.

Peggy Noonan, Who’ll Be 2020’s Margaret Chase Smith?

This Wall Street Journal column should appear at Noonan’s blog today or tomorrow with no paywall.