The progressive magazine The Nation published an open letter last month in which former members of the radical 1960s organization Students for a Democratic Society pleaded with a younger generation of leftists to support Joe Biden for president. The letter, titled “To the New New Left From the Old New Left,” warned that the re-election of President Trump would jeopardize “the very existence of American democracy.”

The signatories expressed fear that some supporters of Bernie Sanders, including members of the Democratic Socialists of America, would “refuse to support” Mr. Biden because they consider him “a representative of Wall Street Capital” — and therefore, in essential respects, not fundamentally better than Mr. Trump.

The letter was fair and sensible in its reasoning and right-minded in its conclusion. Given that the difference of a few thousand votes in states such as Michigan and Wisconsin might allow Mr. Trump to win a second term, a quixotic display of socialist principle in the 2020 election could have disastrous repercussions for the nation and the world.

Unfortunately, the letter’s fears were well-founded. The Democratic Socialists of America had already declined to back Mr. Biden. It has been joined in that refusal by Jacobin magazine, an influential publication among young leftists.