“Senator Warren is deeply qualified to be our next vice president, bringing decades of experience and a track record of leadership on issues from the Green New Deal to fighting corporate greed and corruption — issues that excite the progressive voters you’ll need to win the White House,” the letter says. “Few senators have fought harder and more consistently for the kinds of structural reforms that would lift up working-class families and communities.”

Progressive Democrats of America and RootsAction.org both supported Sanders in the primary. Together, they represent more than 1.2 million activists.

Progressive Democrats of America polled its email list last week on their preference for vice president. With more than 1,000 responses, Warren finished first, with 52 percent support. Sen. Kamala Harris of California finished second, with 18 percent.

“For voters and activists who supported Bernie Sanders in the primary, it’s vital that Biden choose a running mate with a longstanding progressive track record of fighting for the working families of this country,” Alan Minsky, executive director of Progressive Democrats of America, said in a prepared statement.

Outside groups and prominent Democrats have been lobbying Biden’s campaign both publicly and privately on the vice presidential pick since the former vice president became the presumptive Democratic nominee. It is unclear, however, how effective outside efforts will be. Biden is widely expected to make his decision based on personal chemistry and his perception of which running mate could not only help him win, but govern.

Warren is one of several top-tier contenders. She has a small-dollar fundraising machine and a connection to progressive Democrats that could help Biden shore up his support with the party’s most liberal voters. Before he decided not to run for president in 2016, Biden wanted her to be his running mate.

Some moderate Democrats and donors are leery of Warren, and RootsAction co-founder Jeff Cohen said “the main obstacle” for her selection “is the Democratic donor class and Wall Street.”

That opposition, he said, “is exactly why she would make a great running mate, especially in bringing out the progressive voters, the Bernie supporters.”

Warren may face other obstacles, however. Protests since the death of George Floyd in police custody have elevated the prospects of selecting a woman of color. When Sen. Amy Klobuchar of Minnesota took herself out of contention for vice president earlier this month, she publicly encouraged Biden to pick a woman of color.

Harris, Rep. Val Demings of Florida and Atlanta Mayor Keisha Lance Bottoms are among people of color in consideration. VoteVets PAC on Monday released a video making the case for Sen. Tammy Duckworth (D-Ill.), a combat veteran and an Asian American woman.