At the moment, Elizabeth Warren has won more votes on her own in an election than anyone else running for the Democratic Party nomination. Here are some of the numbers:
Warren, US Senate, 2012: 1,696,346
Booker, US Senate, 2014: 1,043,866
Bloomberg, New York Mayor, 2005: 753,090
Biden, US Senate, 2008: 257,539
Sanders, US Senate, 2012: 207,848
Buttigieg, South Bend, Ind., Mayor, 2015: 8,515
Until she dropped out, Kamala Harris had won more votes than anyone else, with 7,542,753 voting for her in 2016 for US Senate.
And one shouldn't forget that Joe Biden won national election as Vice President with 69,498,516 votes in 2008—but he didn't do it on his own.
None of this means much, of course. It's interesting, though, who actually has experience winning millions of votes and who doesn't.