FILE - In this Oct. 19, 2019, file photo Democratic presidential candidate former Vice President Joe Biden and Sen. Elizabeth Warren, D-Mass., participate in a Democratic presidential primary debate hosted by CNN/New York Times at Otterbein University in Westerville, Ohio. Warren, a Massachusetts senator and leading progressive, has become an unlikely confidant and adviser to Biden, the presumptive Democratic presidential nominee. They talk every 10 days or so, according to aides to both politicians who requested anonymity to describe their relationship. (AP Photo/John Minchillo, File)

FILE - In this Sept. 12, 2019, file photo Democratic presidential candidates former Vice President Joe Biden, left and Sen. Elizabeth Warren, D-Mass., talk during a Democratic presidential primary debate hosted by ABC at Texas Southern University in Houston. Warren, a Massachusetts senator and leading progressive, has become an unlikely confidant and adviser to Biden, the presumptive Democratic presidential nominee. They talk every 10 days or so, according to aides to both politicians who requested anonymity to describe their relationship. (AP Photo/David J. Phillip, File)

FILE - In this Feb. 19, 2020, file photo guests watch Democratic presidential candidates, Sen. Elizabeth Warren, D-Mass., and former Vice President Joe Biden speak on a big screen during a Democratic presidential primary debate in Las Vegas, hosted by NBC News and MSNBC. Warren, a Massachusetts senator and leading progressive, has become an unlikely confidant and adviser to Biden, the presumptive Democratic presidential nominee. They talk every 10 days or so, according to aides to both politicians who requested anonymity to describe their relationship. (AP Photo/John Locher, File)