Presumptive Democratic presidential nominee Joe Biden is starting to vet potential vice presidential candidates. Biden has said that his running mate will be a woman, and that she will be selected well in advance of the Democratic convention, now scheduled to begin — in-person or remotely — on Aug. 17.
There is little evidence that vice presidential selections make a difference in presidential election results. The choice of running mate can, however, provide some clues as to how a presidential candidate will approach difficult decisions.
Although the historical evidence of an electoral vote boost from a running mate is not strong, presidential candidates sometimes try to choose someone who will help them win key swing states. However, this is not a universal rule. Dick Cheney and Joe Biden, from states with only three electoral votes, did not help George W. Bush and Barack Obama win electoral votes in 2000 and 2008. Their selections did show that both Bush and Obama wanted Washington insiders as their vice presidents, to compensate for their own relative lack of experience in the nation’s capital.
If party control of the Senate is in the balance, presidential candidates want to choose a running mate who, if currently serving in the Senate, would be replaced by someone from the same political party.
Putting all these considerations together, I believe that Biden’s strongest potential running mates are a group of three senators: Amy Klobuchar, Kamala Harris, and Catherine Cortez Masto. All three represent states with Democratic governors, who would appoint another Democrat to replace them if they were to resign upon being elected vice president. Also, Harris and Klobuchar, as presidential candidates themselves, have already been heavily vetted by the press and political insiders.
Klobuchar is serving her third term in the Senate, having first been elected in 2006. Klobuchar was the district attorney in Minneapolis for eight years before being elected to the Senate, where she serves on the Agriculture, Commerce and Judiciary committees. News reports do indicate that some members of her Senate staff consider her an overly demanding boss. 
Klobuchar has won three Senate elections in Minnesota, a state that Hillary Clinton won by less than 2% of the vote in 2016. In 2018, Klobuchar was re-elected by 24 points. She ran well in both urban and rural areas of the state, and could help Biden not just in Minnesota, but in Wisconsin and Michigan as well.
Kamala Harris is in her first term in the Senate, having been elected to that body in 2016. She started out as an assistant district attorney in San Francisco before becoming that city’s chief prosecutor in 2003. She was elected California’s attorney general in 2010, and served six years in that position. Harris sits on the Budget, Homeland Security and Judiciary committees. 
Harris could help the Biden campaign in several states with increasingly diverse electorates that could turn out to be key swing states this fall: North Carolina, Georgia, Florida and Arizona. 
Catherine Cortez Masto is less well known than others on Biden’s vice-presidential short list. Like Klobuchar and Harris, she is an attorney, and served two terms as attorney general in Nevada. First elected to the Senate in 2016, she sits on the Banking, Energy and Commerce committees, and is chair of the Democratic Senate Campaign Committee for the 2020 cycle. Like Harris, she would help the Democratic ticket in southern and southwestern states with diversifying electorates.
I see Sen. Elizabeth Warren of Massachusetts as a less compelling choice for Biden than Cortez Masto, Harris or Klobuchar. Warren represents a state with a Republican governor, so her selection would put a Senate seat at risk. More importantly, her voters in the Super Tuesday primaries were mostly older, well-educated professionals in coastal states, a constituency that strongly disapproves of Donald Trump and is very likely to vote heavily for Biden regardless of who he selects as his running mate.
Eric L. Davis is professor emeritus of political science at Middlebury College.