Nebraska Attorney General Doug Peterson filed a friend-of-the-court brief Monday in a legal case challenging some Pennsylvania mail-in ballots. 

The case is one of many filed over last week's national election. It concerns a Pennsylvania Supreme Court decision allowing the counting of ballots that were mailed by Election Day and received within three days. 

The Republican Party of Pennsylvania, which filed the challenge, is asking the U.S. Supreme Court to hear it. Several Republican attorneys general across the country are filing amicus, or friend-of-the-court, briefs in support of the Pennsylvania GOP.

Nebraska joined a brief filed by Oklahoma Attorney General Mike Hunter, which argues that the Pennsylvania high court usurped legislative authority to set election rules. Indiana, Kansas, Tennessee and West Virginia also joined in the brief.

"Amici states have important interests in enforcing the absentee ballot deadlines created by their legislatures and ensuring such statutes cannot be amended by state courts merely because some voters will not act in a timely fashion to comply," the brief said.

Peterson's spokeswoman, Suzanne Gage, said he joined in brief "to maintain the proper separation of powers within state governments."

She said he had no response to President Trump's contention that the election is being stolen through election fraud.

The ballots in question have been set aside pending the resolution of legal challenges. They are not included in the unofficial election returns that show Democrat Joe Biden ahead of the GOP president by more than 47,000 votes.

Republican attorneys general from 10 other states, led by Missouri, filed a separate amicus brief Monday. It argues that Pennsylvania overstepped its constitutional authority in accepting late ballots, that voting by mail creates voter fraud risks and that the decision to accept ballots after Election Day exacerbated risks of absentee ballot fraud.