Maine Secretary of State Matthew Dunlap, one of four Democrats appointed to the 11-member commission, says he filed the lawsuit Thursday. It claims violations of the Federal Advisory Committee Act, which requires commissioners to be given all materials prepared for or by the commission.
Dunlap says multiple requests for information sent to commission staff have been ignored or rebuffed.
The Trump commission has stirred controversy from the moment it was established last spring. Critics say Trump is using it to find support for his unsubstantiated claims of widespread voter fraud that cost him the popular vote during the 2016 election.