"Split vote" refers to a situation where people divide their votes among different choices instead of all supporting one option. This can result in no clear winner and make it difficult for any single choice to gain a majority. Full definition
Similar things happen in politics when two parties appeal to the same kind of voters, splitting their votes between them and allowing a third party unpopular with the majority to win the election. (newscientist.com)
He insisted, however, he wasn't in the race to simply split votes with his hated rival. (observer.com)
Then you shouldn't have split your vote by supporting 3rd party candidates. (nystateofpolitics.com)