“I have seen Ms. Warren in small church basements and in packed gymnasiums,” Ms. Pressley said in the video. “And she is consistent. She never loses sight of the people.”

“You’ve all heard about the senator’s plans — but here’s the thing,” she added. “Her plans are about power: who has it, who refuses to let it go, and who deserves more of it.”

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The endorsement was a must-have for Ms. Warren, who has risen sharply in polls and fund-raising over the last six months but has lagged in some traditional measures of party support, including endorsements from governors, big-city mayors, or fellow senators outside Massachusetts. In recent weeks her campaign has sought to reverse that trend by lining up endorsements from anti-establishment progressive champions popular among the party’s left wing.

Representative Katie Porter of California endorsed Ms. Warren over her home-state legislator, Senator Kamala Harris of California. Mayor Jim Kenney of Philadelphia announced his support for Ms. Warren’s campaign last month, her first endorsement from a big-city mayor.

Ms. Warren also recently rolled out endorsements from two of the country’s most progressive district attorneys: Larry Krasner of Philadelphia and Rachael Rollins of Suffolk County in Massachusetts.

Mr. Krasner, who supported Mr. Sanders in 2016, received some blowback for endorsing Ms. Warren over Mr. Sanders, who remains in striking distance in most polls.

“I have not forgotten and no one should forget that Bernie Sanders, early in his prior campaign for president, said the only reason he ran was that he couldn’t persuade his friend Elizabeth Warren to run for president,” Mr. Krasner said in response. “I heard what he said, and I remembered what he said.”