Other forms: swing voters
A swing voter is someone who hasn't yet decided how they'll cast their ballot in an upcoming election. Swing voters aren't loyal to one particular political party.
Politicians work hard to entice swing voters to vote for them. Unlike people who always cast their votes for members of a specific party, it's hard for pollsters to guess which way swing voters will lean in an election. Someone who votes for a Republican senator one year and a Democratic presidential candidate two years later would be considered a swing voter. And a voter who remains undecided until the very last minute is also a swing voter.