A coalition of liberal groups is calling on former vice president Joe Biden and Sen. Bernie Sanders to choose a woman of color as their running mate, as a way to boost turnout among the key groups of voters Democrats need to win the presidency.

In a letter sent to both candidates, the last two viable contenders in the Democratic primary, the groups note that women and people of color made up the majority of the party’s voters in the past two presidential elections. “At the intersection of those communities … women of color have a unique and critical role to play in galvanizing the entire progressive coalition,” the letter reads.

Biden pledged during Sunday’s Democratic debate to pick a woman as his running mate if he wins the nomination. Sanders (I-Vt.) said he would “in all likelihood” choose a woman.

Aimee Allison, founder of She the People, said the letter is “a warning to not repeat the mistake that Hillary Clinton’s campaign made in 2016” by tapping a white man as her running mate.

“The warning is, if you have an all-white ticket, if you don’t put a woman of color on the ticket, some of us will come out for you, but not in the numbers you need to win the battleground states,” Allison said.

Donald Trump won narrow victories in states where turnout among traditional Democratic voters was down, including Pennsylvania and Wisconsin. Clinton struggled to excite younger voters, along with African Americans and Latinos, after a bitter primary battle with Sanders. It was later revealed that Russian disinformation campaigns on social media targeted black voters and might have contributed to lower turnout.

Female black voters were Clinton’s strongest supporters, with 94 percent voting for her, while 52 percent of white women voted for Trump.

The letter does not include the names of any potential vice-presidential picks but states that “there are numerous talented women of color to choose from.” It also says: “This choice is a first indication of how you will govern, and we want to know it will be in partnership with the constituencies that comprise the vast majority of Democratic voters.” In addition to being led by She the People and Democracy in Color, the letter is signed by the Collective PAC, Indivisible, Latino Victory Project, New Florida Majority, New Georgia Project Action Fund, New Virginia Majority, PowerPAC and Southern Elections Fund.

Biden, who served under the nation’s first black president, Barack Obama, now leads the delegate count, thanks to strong support from black voters, particularly older black women.

Sanders has done better with young voters, including black and Latino voters. But just as he did in his 2016 bid, the senator has struggled to gain support from older African American women.

“I think both candidates stand to gain by choosing a woman of color as their vice president,” said Andrea Mercado, executive director of the New Florida Majority, which works to educate and mobilize voters of color in the crucial swing state. She also said the pick should be “not just any woman of color, but someone who is a progressive woman of color.”

In a state like Florida, where the past two presidential elections have been decided by just over or just under a single percentage point, Mercado said it is crucial that Democrats choose a candidate who can excite the state’s voters of color.

“We know that candidates matter and the swing voters aren’t just only … those who swing from red to blue, but also those who stay home when they’re not excited or energized,” she said.

Correction: An earlier version of this report misidentified the founder of She the People as Aimee Alexander. She is Aimee Allison.