Sen. Bernie Sanders, I-Vt., greets the crowd as he arrives for a campaign rally in Tulsa, Okla., in 2016, when he built a large political movement during his bid for the Democratic presidential nomination. Many in the party have adopted parts of his agenda, even though candidates he backs mostly have lost in 2018.

Sen. Bernie Sanders, I-Vt., greets the crowd as he arrives for a campaign rally in Tulsa, Okla., in 2016, when he built a large political movement during his bid for the Democratic presidential nomination. Many in the party have adopted parts of his agenda, even though candidates he backs mostly have lost in 2018.