undervote

under- +‎ vote

undervote (plural undervotes)

1. A vote that has been cast but that shows no legally valid preference for a candidate in an election.
2. The number of such votes in an election.

• overvote

undervote (third-person singular simple present undervotes, present participle undervoting, simple past and past participle undervoted)

1. (politics) To spoil a ballot by marking it for fewer choices than mandated by the rules.

• undervote on Wikipedia.Wikipedia

• CNN Fact check: Examining Florida's 'undervote'

• overtuned