With party leaders focused on Mr. Trump, young progressives like Rep. Alexandria Ocasio-Cortez have taken on the task of making the party more left-wing. She “has had the uncommon ability for a first-year member of Congress to push the debate inside the Democratic party to the left,” the New York Times reported last week.

Ms. Ocasio-Cortez’s talk of a 70% marginal tax rate on the highest incomes, a “Green New Deal” to replace fossil fuels within 10 years, and abolition of the Immigration and Customs Enforcement agency has attracted media attention and favorable comments from Democratic presidential candidates. Yet these are radical ideas unlikely to help the party win in 2020.

Older Democrats haven’t stopped Ms. Ocasio-Cortez and her progressive allies from becoming a significant force in shaping their party’s future. They’ve been too busy resisting Mr. Trump.