“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.”…

Some Sanders primary voters said they would back a third-party candidate, a few said they would vote for Mr. Trump, and some were wavering about voting at all. Daniel Ray, 27, of Lancaster, Penn., planned to write in Tulsi Gabbard, the congresswoman from Hawaii who ended her long-shot bid for the Democratic nomination last month.

In Facebook groups for Sanders supporters and on Twitter, cynicism persisted about the olive branches on policy that Mr. Biden has offered to progressives and, at the extreme, some accused Mr. Sanders of selling out his leftist movement.