General elections

Democratic presidential candidate former Vice President Joe Biden speaks to members of the press at the National Constitution Center in Philadelphia, Tuesday, March 10, 2020. (AP Photo/Matt Rourke)

Democratic presidential candidate former Vice President Joe Biden, accompanied by his wife Jill, speaks to members of the press at the National Constitution Center in Philadelphia, Tuesday, March 10, 2020. (AP Photo/Matt Rourke)

Democratic presidential candidate former Vice President Joe Biden enters a campaign event in Columbus, Ohio, Tuesday, March 10, 2020. (AP Photo/Paul Vernon)

FILE - In this Nov. 6, 2018 file photo, Sen. Claire McCaskill, D-Mo., steps on stage to deliver a concession speech in St. Louis. The two-term senator from Missouri lost her seat in the 2018 midterm election but is now making waves as a plainspoken analyst for NBC News. (AP Photo/Jeff Roberson, File)

FILE - In this Jan. 3, 2019, file photo, House Speaker Nancy Pelosi of Calif., right, poses during a ceremonial swearing-in with Rep. Henry Cuellar, D-Texas, right, on Capitol Hill in Washington. Last week's Super Tuesday proved a tough day for progressive Democratic candidates. After narrowly thwarting a youthful progressive upstart's primary challenge, veteran Cuellar says his fellow Democrats greeted him on the House floor last week with one overwhelming sentiment _ gratitude. (AP Photo/Cliff Owen, File)

Democratic presidential candidate U.S. Sen. Bernie Sanders, I-Vt., speaks during a campaign rally at the University of Michigan in Ann Arbor, Mich., Sunday, March 8, 2020. (AP Photo/Paul Sancya)

Democratic presidential candidate Sen. Bernie Sanders, I-Vt., is hugged as he works the crowd after a campaign rally in Chicago's Grant Park Saturday, March 7, 2020. (AP Photo/Charles Rex Arbogast)

Volunteers hand out signs at a rally for Bernie Sanders at Calder Plaza in Grand Rapids, Michigan on Sunday, March 8, 2020. Michigan's presidential primary is Tuesday.(Anntaninna Biondo/The Grand Rapids Press via AP)

FILE - In this July 31, 2019 file photo, Sen. Cory Booker, D-N.J., gestures to former Vice President Joe Biden during the second of two Democratic presidential primary debates hosted by CNN in the Fox Theatre in Detroit. Booker of New Jersey has endorsed Joe Biden. Booker announced on Twitter early Monday that Biden will “restore honor to the Oval Office and tackle our most pressing challenges.” Booker ended his own presidential bid in January, pledging to do “everything in my power to elect the eventual Democratic nominee for president." (AP Photo/Paul Sancya)

FILE - In this Sept. 20, 2019, file photo, Democratic presidential candidate Montana Gov. Steve Bullock speaks during the Climate Forum at Georgetown University, in Washington. Bullock, who insisted he won’t run for the U.S. Senate, is poised to do just that, according to a person familiar with his plans but unauthorized to discuss them and granted anonymity. (AP Photo/Jose Luis Magana, File)