While the swing voter is ostensibly the target of most political activity during elections, in countries without compulsory voting the political parties know that the shift from one party to another is dependent only to a limited extent on swing voters. (Wikipedia)

In a country where many voters identify with a particular political party, "swing voters" -- those who do not have a strong party affiliation -- often take center stage in an election year (Online News Hour)

The only time our vote really matters is when it breaks a tie or creates a tie – when our vote becomes the swing vote (scienceline.org)