At close to 60,000, the D.S.A.’s national membership is considerable, but not nearly as large as more mainstream political organizations on the left like MoveOn.org or Emily’s List, which have millions of members. Still, it is seen as an increasingly potent force in American politics, particularly after helping elect Alexandria Ocasio-Cortez to Congress in 2018.

And even without endorsing a candidate in the general election, the D.S.A. has committed to keeping socialist ideas in circulation throughout the 2020 campaign. A statement on its home page reads: “Bernie Sanders launched a political revolution and D.S.A. continues to build it, training our chapters to effectively support democratic socialist candidates running for local and state office while lifting up our key issues.”

Matt Hill, a spokesman for the Biden campaign, declined to comment on the D.S.A.’s decision not to support the former vice president’s candidacy. But Hill pointed out that in recent weeks Biden adopted a range of left-leaning policies, including making public colleges and universities tuition-free for those with a family income under $125,000 and lowering the Medicare eligibility age to 60.

“We feel we need the support of progressives to build the strongest possible coalition,” Hill said. “That’s why we’re engaging progressives, young Americans, climate and racial justice activists to help us win in November.”