Republican Eric Hovde was behind in a near-deadlocked race with incumbent Democrat Tammy Baldwin for a hotly contested Senate seat.
With 99% of the vote counted, Baldwin was leading with just over 35,000 votes. More than 3 million votes were cast in the election.
Baldwin declared victory in the race but absentee ballots had not been counted in Oak Creek and all votes had not been counted in Oshkosh, according to the Milwaukee Journal-Sentinel.
"It is clear that the voters have spoken and our campaign has won," Baldwin said in a statement early Wednesday. "The people of Wisconsin have chosen someone who always puts Wisconsin first, someone who shows up, listens, and works with everyone to get the job done."
The race attracted national attention and a lot of spending as Baldwin's seat was considered vulnerable.
Hovde backers spent much of the final days of the election with ads going after Baldwin's same-sex partner, Maria Brisbane, a private wealth adviser. Hovde had tried to frame their relationship as a potential conflict of interest but critics claimed the tactic was an attack on her support of LGBTQ rights.
Hovde can request a recount because his loss was by less than 1%.
Donald Trump won the state by about 30,000 votes over Kamala Harris.
