There was a time not so long ago when the young seemed destined to be liberal forever. Americans in their teens and 20s were to the left of their elders on social issues. They worried more about poverty. They voted strongly Democratic.

In retrospect, we refer to this period as the 1960s, and it didn’t last long, let alone forever. Less than a generation after young people were marching for civil rights and against the Vietnam War, they voted overwhelmingly for Ronald Reagan.

Today, of course, the young are liberal again, and it seems as if they will be forever. They favor same-sex marriage, marijuana legalization, stricter gun laws, citizenship for illegal immigrants and an activist government that fights climate change and inequality. The Republican Party, as you have probably noticed, does not.

But the temporary nature of the 1960s should serve as a reminder that politics change. What seems permanent can become fleeting. And the Democratic Party, for all its strengths among Americans under 40, has some serious vulnerabilities, too.