The lawsuit, filed against Minnesota Secretary of State Steve Simon, follows a similar state court lawsuit filed last week by the Minnesota Alliance for Retired Americans Educational Fund.
Both cases come as Simon and other state election officials race to prepare for an expected surge in absentee voting this year.
Simon declined to comment on the lawsuits Tuesday. He has urged Minnesota voters to cast their ballots by mail this year to prevent spreading the virus.
A $17 million compromise election bill signed into law last week set aside money to encourage voting by mail and to help the state process an expected rise in such voting. But an effort by Simon to send all registered voters mail-in ballots before the August primary and November general elections was rejected by Republicans in the Legislature.
More than 22,000 people applied to receive absentee ballots on the first day an online application tool became available, according to Simon's office.But the federal lawsuit filed Tuesday argues that Minnesota's witness signature requirement makes the state's election system "especially vulnerable to the risk of suppressed voter participation during the COVID-19 pandemic." The suit also seeks to invalidate a requirement that the witnesses be registered to vote in Minnesota. The League says that can be too restrictive for Minnesota voters trying to cast ballots from out of state.
According to the Campaign Legal Center, which sued on behalf of the League of Women Voters, Minnesota is one of 12 states to require witness or notary signatures on absentee ballots. Two states — Virginia and Rhode Island — have made changes to such requirements because of the virus.