Alexandria Ocasio-Cortez’s surprise primary victory over Rep. Joe Crowley seems likely to hurt Congress, America and the Democratic Party. It doesn’t have to.

Because the policies Ms. Ocasio-Cortez advocates are so far from the mainstream, her election in November would make it harder for Congress to stop fighting and start fixing problems. Thanks to a small percentage of primary votes, all of the people of New York’s 14th Congressional District stand to lose a very effective representative in Washington.

Fortunately,...