floating voter 
noun British
A person who has not decided which way to vote in an election, or one who does not consistently vote for the same political party: the party leader stepped up his efforts to appeal to floating voters...
- Election features include BBC Southern Counties Radio following the opinions of a panel of floating voters and an election soap opera at BBC Radio Leicester.
- The Labour Party is an electoral-machine and as such chases the votes of floating voters in middle England.
- In particular, the parties are interested in those voters in the marginal constituencies who are likely to switch parties - the floating voters.