Exit polls might be one way to get an estimate. "Who did you vote for? Did having Jane Doe for a running mate influence your vote?"
I think they tried that on the last election and then some.....
ABC News, CBS News, CNN and NBC News are all part of the National Election Pool, which is run by Edison Research. According to Edison, it has conducted the only exit polls in the U.S. since 2004.
NEP, based in New Jersey, says it
conducts in-person interviews with over 100,000 voters as they leave their polling stations on the day of the election. The group also interviews voters at early voting locations, as well as phone interviews with early and absentee voters.
When exit polls show one candidate with a massive lead that is well beyond the surveys' margin of error, the networks in the NEP make early projections calling that candidate the winner.
But exit polls can get it wrong.
In the 2016 presidential election, exit polls appeared to indicate some battleground states were breaking for Hillary Clinton, but they ended up being won by Donald Trump.
Similarly, in 2004, exit polling appeared to indicate that the Democratic challenger John Kerry was poised to defeat incumbent President George W. Bush, who went on to win reelection.
And in the 2008 New Hampshire Democratic primary, exit polls indicated Barack Obama was going to beat Clinton, who went on to win the state.
In perhaps the most famous case, both Al Gore and George Bush were declared the winners of the key state of Florida in the 2000 election, in a race that was actually too close to call and was not decided until a Supreme Court ruling a month later.
Super Tuesday: How are races called with 0% of vote reporting?