In the months since Mr. Sanders dropped out of the race, effectively making Mr. Biden the nominee and starting the clock for the vice-presidential selection, progressives have had a string of victories further down the ballot. Challengers backed by groups like Sunrise and Justice Democrats won in New York and Illinois, including in some races against prominent House incumbents. Each member of the so-called Squad, the group of progressive women of color in the House who have at times rankled House leadership, secured re-election with comfortable margins.
The biggest victory came in St. Louis, when Ms. Bush unseated a longtime congressman, Representative William Lacy Clay, and provided a new playbook for how to appeal to Black voters with the progressive platform.
The growing grass-roots power of these progressives, if sustained, could set up a clash with Mr. Biden and Ms. Harris, who have a shared instinct for political caution. During the primary campaign both were targeted relentlessly by the progressive left but rarely budged. Mr. Biden tailored his message to focus on how he could defeat Mr. Trump, sidelining policy as a focus. Ms. Harris moved in the opposition direction, rejecting “Medicare for all” after initially co-sponsoring Mr. Sanders’s bill on the health care plan.
This leftward shift could change the dynamics in Washington, said Mondaire Jones, a lawyer who won the primary in New York’s 17th Congressional District and is most likely headed to Capitol Hill in the fall. Mr. Jones praised Ms. Harris’s selection as historic, but suggested that liberals would continue to press their agenda.
“The left in recent years has never shied away from a fight with members of the Democratic Party, and I see no reason why we won’t flex our newfound muscle in a Biden-Harris administration,” Mr. Jones said.
“Biden and Harris would be working with the most progressive Congress we’ve seen,” he said. “And I’m ready to make sure they’re sticking to their promises and will be part of the effort to push them to be more in line with progressive values.”
Among liberals, the acceptance of Ms. Harris, if reluctant, still amounted to a surprising about-face after months of lobbying for her more progressive rivals to appear on the ticket.