The Obama campaign and the Democratic National Committee are suing to change voting laws in the battleground state of Ohio.
They want to overturn a provision that gives members of the military three extra days of early voting. It would allow them to vote until the Monday right before an election, while the cutoff for early voting for everyone else is three days earlier.
The Obama camp argued in its lawsuit that giving men and women in uniform more time than other voters is "arbitrary" and unconstitutional. They say their lawsuit is about ensuring equal voting rights for everyone else.
Fifteen pro-military groups are suing to dismiss Obama's suit. They argue that military voters face obstacles that other voters don't face.
Mitt Romney issued a statement on Facebook calling President Obama's lawsuit "an outrage."
"If I'm entrusted to be the commander-in-chief, I'll work to protect the voting rights of our military, not undermine them," he said.