The Wisconsin high court affirmed a ruling from a lower court judge after a conservative group, the Wisconsin Institute for Law & Liberty, sued the state's elections commission on behalf of two voters.
The commission had approved the use of drop boxes in response to the pandemic, when many voters were anxious to limit in-person interactions. The November 2020 election included 528 boxes statewide, according to election officials.
But Justice Rebecca Bradley, writing for the majority, said that under state law ballots must be returned to a clerk's office or another designated site, not an "inanimate object" such as an unstaffed box.
"Only the legislature may permit absentee voting via ballot drop boxes," she wrote.