In an election, a protest vote is a vote against the party you usually support in order to show disapproval of something they are doing or planning to do.
protest vote in British English
(ˈprəʊtɛst vəʊt)
noun government
1.
a vote cast for someone other than the candidate who might have expected it, in order to show the voter's discontent
Can they afford to make a protest vote without dire political consequences?
2.
body of protest votes (or voters) collectively
the party's slick grassroots machine pulling in the protest vote
Opposition parties do well from the protest vote.
Examples of 'protest vote' in a sentence
protest vote
Please be careful what you wish for when you cast a protest vote.
Times, Sunday Times (2014)
I'm going to cast a protest vote.
Times, Sunday Times (2011)
The British people regard the European elections as a protest vote.
Times, Sunday Times (2014)
And in a Euro election in which many voters cast protest votes for fringe parties, no mainstream party leapt forward.