More than 100 liberal activists, leaders and celebrities signed a letter urging Joe Biden to select Sen. Elizabeth Warren as his running mate, intensifying pressure on the presumptive Democratic nominee from the left as he faces competing demands to pick a black woman.

The letter portrays Warren (D-Mass.) as the best prepared prospect to serve as president and one uniquely capable of helping Biden politically in the November election. It asserts that he is “already strong” among nonwhite voters but could use help winning over disaffected voters who backed Sen. Bernie Sanders (I-Vt.) in the primary — even as some of them have soured on Warren.

“A crisis election as big as 1932 requires a big running mate. So why not the best?” says the letter, which bears the names of an array of left-leaning figures ranging from actress Jane Fonda to leading activists such as Ady Barkan and Charles Chamberlain.

It adds, “Elizabeth Warren has proven herself most prepared to be President if the occasion arises and deeply expert on the overlapping emergencies now plaguing America — Covid-19, Economic Insecurity, Racial Injustice and Climate Change.”

The letter, sent to Biden’s campaign on Friday, underlines the dueling pressures the former vice president is facing as he weighs his choices. While many on the left favor Warren, the nationwide protests over racism and police violence have prompted growing calls for Biden to choose an African American woman.

This has added a challenge for white candidates such as Warren, who lack deep ties to African American communities, some Biden allies believe. As a candidate for president, Warren attracted mostly white crowds to her events and struggled to break through with black voters.

Laurence Tribe, a constitutional law professor at Harvard University, where Warren once taught, said that there would be some “symbolic ways in which some people would be disappointed” if Biden does not choose an African American woman, and that disappointment should count. But Warren’s record, he said, makes her the strongest choice.

“I think African Americans above all would be the first to say they are more interested in results than cosmetics,” said Tribe, who signed the letter.

But many black activists disagree and are advocating for several alternatives to Warren. At least several African American women have advanced to the next stage of Biden’s search.

The letter comes as Biden’s undertaking has advanced to the next phase, with his campaign conducting more extensive reviews of some of the prospects it initially identified. Warren is among the group of candidates who have progressed to the point of more comprehensive vetting, according to people with knowledge of the situation.

Among the candidates who have progressed to that stage or have the potential to do so are Sen. Kamala D. Harris (D-Calif.), Rep. Val Demings (D-Fla.), former national security adviser Susan E. Rice and Atlanta Mayor Keisha Lance Bottoms, all of whom are black.

Along with Warren, New Mexico Gov. Michelle Lujan Grisham, who is Latina, is also in that group. The pool of prospects remains fluid, and some close Biden allies suggested other contenders could also face the more intensive vetting process.

Allies of Warren are trying to underscore her commitment and accomplishments on racial justice and the letter is in part designed to bolster her credentials. It says that Warren’s “scholarly studies explained how economic fraud and banking practices especially devastated families of color” as well as “what to do about that racial injustice.”

About a fifth of those who signed the letter are people of color, according to former New York City public advocate Mark Green, who helped spearhead the effort.

Beyond Warren’s policy résumé, the letter tries to get the Biden campaign to envision her taking on Vice President Pence in a debate this fall. Warren had some standout debate moments during the Democratic primary.

“She’s a very skilled communicator and debater,” it says. “Imagine her on stage debunking Mike Pence or needling ‘President Tweety.’ ”

The letter says Warren holds a political advantage that others do not — the ability to help persuade Sanders voters who might not be enthusiastic about Biden.

“Anyone can speculate about who could most help your ticket politically in swing mid-western states or with people of color, both areas where you’re already strong,” it says. “Another big variable is who could persuade millions of disappointed Sanders supporters to vote for your ticket.”

Although Warren and Sanders hold similar policy views, many Sanders supporters were upset that Warren did not endorse the senator from Vermont when she exited the primary, instead staying neutral until it was effectively settled with Biden emerging.

To this day, there remains some animosity from Sanders backers. Sanders himself is not publicly advocating for a specific individual to be named as Biden’s running mate.

Former New York City public advocate Mark Green helped spearhead the letter and sent it to the Biden campaign on Friday. He said he did not coordinate with Warren or her office on it. The Biden campaign did not have any immediate comment.