Optimism filled the Prospect Park air as more than 100 supporters of Senator Elizabeth Warren’s presidential campaign gathered last week for an organizing event called “How to Be All In for Elizabeth.” But as dusk approached, the final “She Has a Plan” button was handed out, and the crowd of mostly millennial Brooklynites began to disperse, hope gave way to skepticism.

Michael Rose, 36, expressed concern that a candidate like former Vice President Joseph R. Biden Jr. would earn the party’s nomination, and that Democrats, faced with a historically diverse field, would choose “an old white guy that appeals to moderates.”

Amelita Lijek, 31, said the primary represented a crossroads for the party. “We have this choice here: to go positive and go with what feels inspiring, or to live in doubt and cower,” Ms. Lijek said. “It’s terrifying.”

After months of liberal senators dominating the Democratic presidential primary, and leftist ideas such as the Green New Deal and “Medicare for All” getting significant national attention, Mr. Biden’s entrance into the race last month has swung the conversation back toward the center.