FILE - In this March 2, 2020 file photo, Sen. Amy Klobuchar, D-Minn., endorses Democratic presidential candidate former Vice President Joe Biden at a campaign rally Monday, March 2, 2020 in Dallas. As presumptive Democratic presidential nominee Joe Biden begins the process of choosing a running mate amid the coronavirus crisis, managing the pandemic has become its own version of an audition. For potential picks, lobbying for the job means breaking into the national conversation, positioning themselves as leaders and executing at their day job. (AP Photo/Richard W. Rodriguez)

FILE - In this Jan. 16, 2020 file photo, Sen. Kamala Harris, D-Calif., talks to reporters about the impeachment trial of President Donald Trump on charges of abuse of power and obstruction of Congress, at the Capitol in Washington. As presumptive Democratic presidential nominee Joe Biden begins the process of choosing a running mate amid the coronavirus crisis, managing the pandemic has become its own version of an audition. For potential picks, lobbying for the job means breaking into the national conversation, positioning themselves as leaders and executing at their day job. (AP Photo/Matt Rourke)

FILE - In this March 2, 2020, file photo Sen. Amy Klobuchar, D-Minn., endorses presumptive Democratic presidential candidate former Vice President Joe Biden at a campaign rally in Dallas. Klobuchar praises the former vice president on television and Twitter as she pushes her plan for an all-mail November election and responding to coronavirus in rural America. (AP Photo/Eric Gay, File)

FILE - In this July 31, 2019 file photo, former Vice President Joe Biden speaks as Sen. Kamala Harris, D-Calif., listens during the second of two Democratic presidential primary debates hosted by CNN in the Fox Theatre in Detroit. As presumptive Democratic presidential nominee Joe Biden begins the process of choosing a running mate amid the coronavirus crisis, managing the pandemic has become its own version of an audition. For potential picks, lobbying for the job means breaking into the national conversation, positioning themselves as leaders and executing at their day job. (AP Photo/Paul Sancya)

FILE - In this Jan. 29, 2020, file photo, Michigan Gov. Gretchen Whitmer delivers her State of the State address to a joint session of the House and Senate, at the state Capitol in Lansing, Mich. As presumptive Democratic presidential nominee Joe Biden begins the process of choosing a running mate amid the coronavirus crisis, managing the pandemic has become its own version of an audition. For potential picks, lobbying for the job means breaking into the national conversation, positioning themselves as leaders and executing at their day job. (AP Photo/Al Goldis, File)

FILE - In this Sept. 12, 2018 photo, Former Vice President Joe Biden takes a photo with Samantha Green and daughter Ava Green during a visit with Michigan Democratic gubernatorial candidate Gretchen Whitmer, left, and running mate Garland Gilchrist to Leo's Coney Island in Southfield, Mich. As presumptive Democratic presidential nominee Joe Biden begins the process of choosing a running mate amid the coronavirus crisis, managing the pandemic has become its own version of an audition. For potential picks, lobbying for the job means breaking into the national conversation, positioning themselves as leaders and executing at their day job. (Daniel Mears/Detroit News via AP)

FILE - In this Feb. 26, 2020 file photo, Democratic presidential candidate Sen. Amy Klobuchar, D-Minn., speaks during a campaign event in Charleston, S.C. As presumptive Democratic presidential nominee Joe Biden begins the process of choosing a running mate amid the coronavirus crisis, managing the pandemic has become its own version of an audition. For potential picks, lobbying for the job means breaking into the national conversation, positioning themselves as leaders and executing at their day job. (AP Photo/Matt Rourke)

FILE - In this Nov. 15, 2019, file photo former Georgia House Democratic Leader Stacey Abrams, speaks at the National Press Club in Washington. Presumptive Democratic presidential nominee Joe Biden has already narrowed the field by saying he will pick a woman. In addition to Sen. Kamala Harris, Sen. Amy Klobuchar and Michigan Gov. Gretchen Whitmer, other names that have been part of the speculation are Sen. Elizabeth Warren of Massachusetts and Abrams. (AP Photo/Michael A. McCoy, File)

FILE - In this Dec. 19, 2019, file photo presumptive Democratic presidential candidates Sen. Elizabeth Warren, D-Mass., left, and former Vice President Joe Biden stand on stage during a break at a presumptive Democratic presidential primary debate in Los Angeles. (AP Photo/Chris Carlson, File)