U.S. voters getting early jump at the polls

Political parties generally have access to regularly updated government records showing when someone votes, enabling them to time their phone calls and mailings accordingly or give an extra nudge to voters who asked for mail-in ballots but haven't yet returned them.

"It's expensive and it's time-consuming, so a campaign that is just getting going and has a huge influx of money late, it may be hard for him to get ramped up to do that work if he's not already on the ground," Gronke said. "It will benefit the better organized, better funded campaigns."

Early votes don't get counted until election day, but there is nothing to stop campaigns from calling those who have already cast ballots and asking who they voted for. Released publicly, such exit poll results could help sway voters who have yet to cast ballots, creating a self-fulfilling prophecy that raises ethical questions, Gronke said.

But the downside of early voting in a primary election is the risk of voting for a candidate who withdraws before that vote is counted, notes Quinnipiac University pollster Peter Brown.

"In a general election, voting early, you know who the candidates are," Brown said. "In this system, it's essentially a knockout system ... I think we all know the field will be narrower."

(Additional reporting by Adam Tanner in San Francisco; Editing by Eric Beech)