Who will be Joe Biden’s running mate?

Given Biden’s age, voters will be tempted to view his vice-president as a future president or, at least, a major contender for the top job one day. If Biden wins and decides not to seek re-election, his second-in-command would be in the catbird seat for 2024.

Throughout history, it’s hard to handicap vice-presidential derbies without knowing the innermost feelings of the decision-makers. What does Biden really think of Kamala Harris, Elizabeth Warren and Amy Klobuchar? He’s campaigned against them and with them. He’s sized up their strengths and weaknesses. We have no idea what he really thinks.

Over the years, Biden has had dealings with others, such as former Arizona governor and cabinet member Janet Napolitano, Sens. Tammy Duckworth of Illinois and Tammy Baldwin of Wisconsin. He’s followed the careers of Govs. Gretchen Whitmer of Michigan and Michelle Lujan Grisham of New Mexico, Florida Rep. Val Demings and former Georgia state legislator Stacey Abrams. About them, too, his true feelings are a mystery.

Presidents want vice-presidents they trust.

Another unknown is the vetting process. Probing the backgrounds of Democratic VP contenders by Biden’s inner circle could unearth disqualifying flaws that we now know nothing about.

Vice-presidential screenings, as always, should start with capacity to be president. While that metric is supposedly used, and sometimes really is, political factors often push it aside.

Can the running mate deliver a swing state? In close elections, that counts. Virginia Sen. Tim Kaine was less than a stellar national candidate, but he did help Hillary Clinton bag his state’s 13 electoral votes––Virginia was the only southern state she carried. Bill Clinton’s number two, Al Gore, played a big role delivering Tennessee twice.

Running mates don’t always bring their states with them. Paul Ryan failed to carry Wisconsin for Mitt Romney, just as John Edwards couldn’t win North Carolina for John Kerry.

Balance can have strategic importance. Biden, himself, was picked by Barack Obama because his foreign policy credentials and long Senate service balanced Obama’s inexperience. The frumpy Lyndon Johnson, a Protestant from Texas with legendary legislative skills, was picked to balance the glamorous, 43-year-old John Kennedy, a Catholic from Massachusetts.

But Bill Clinton proved that balance isn’t always desirable. His choice of Al Gore––another young, Ivy League, southern centrist––was an exercise in image reinforcement.

Energizing the party’s base becomes a factor when the presidential nominee needs to shore up grassroots activists. John McCain’s choice of Sarah Palin and Donald Trump’s selection of Mike Pence were designed to reassure staunch conservatives without offending swing voters. McCain’s pick flopped, Trump’s succeeded.

Campaign experience is a consideration––having a running mate who’s already been through the paces of a national campaign is a big plus. That helped Gore and George H. W. Bush, two losing presidential contenders, win VP nods. Then there is political obligation––Franklin Roosevelt put John Nance Garner on the ticket in return for his delegates at the 1932 convention.

In the current vice-presidential sweepstakes––Warren and Napolitano may be seen as best prepared to be president, but others could lay claim to that mantle as well. In terms of delivering a critical state, Whitmer tops the list with Michigan’s 16 electoral votes––although Klobuchar could help with Minnesota and Baldwin with Wisconsin. Being from Florida, the quintessential battleground, is a plus for Demings; her downside is that she’s only won one of the state’s 27 House districts and has never won a statewide election.

Warren on the ticket, for sure, would solidify the party’s left wing around Biden’s candidacy––but she could also turn off moderates and swing voters. Harris, Klobuchar and Warren all have recent national campaign experience, which is a significant strength, but Whitmer, a 48-year old midwestern governor, may best balance Biden’s traits.

How about diversity? Biden could not have won the nomination without steadfast support from African Americans. Democrats who want a diverse ticket now handicap Kamala Harris as the strongest choice––although, in truth, she had trouble attracting support from black voters in her own presidential bid. That’s why Demings and Abrams, and perhaps others, may still be in the hunt.

Interestingly, two of the most popular possibilities for vice-president are off the list––Michelle Obama, who doesn’t want it, and Gov. Andrew Cuomo of New York, who has been ruled out based on Biden’s pledge to select a woman.

While Joe Biden’s decision is the source of frantic speculation, and could make history, it probably won’t determine who wins in November. Voters vote for president, not vice president. Although this time, with a 77-year old candidate, the choice takes on added importance.

A version of this column was published in The Times Picayune, see here