Mary Shippee voted for Senator Bernie Sanders in Wisconsin’s Democratic primary this month, well after it was clear he had no chance to become the party’s presidential nominee.

Now that Mr. Sanders has dropped out and endorsed former Vice President Joseph R. Biden Jr., Ms. Shippee is torn over whether to once again cast a vote for a moderate Democrat in November, after grudgingly supporting Hillary Clinton in 2016 and President Barack Obama in 2012.

“What it feels like is the Democratic Party relies on guilting progressives into voting for them, and they don’t want to have any meaningful changes,” said Ms. Shippee, 31, a nursing student in Milwaukee. “For the third election in a row, to have a candidate you’re not excited about makes me a little more interested in voting third party.”

Despite Mr. Sanders’s call to unite behind Mr. Biden to defeat President Trump — whom the Vermont senator described as “the most dangerous president” of modern times — and despite Mr. Obama’s assurance that the party had moved left since he left office, the youthful and impassioned army of Sanders supporters is far from ready to embrace a nominee so unlike the one they pinned their dreams on.