LAS VEGAS — The Nevada Democratic Party is siding with the state in a legal tussle over how to fill a vacant U.S. House seat.

Lawyers representing Democrats in Nevada and Washington submitted a legal brief Thursday opposing a state Republican party lawsuit that challenges the rules of an upcoming special election.

Under state statute, there will not be a primary.

Democratic Secretary of State Ross Miller previously said the state's first special election to fill a House vacancy will be determined by an open contest.

The Nevada Republican Party lawsuit claims political parties should pick nominees to be listed on the special election ballot. It also demands a court injunction to prevent the state from allowing more than one candidate from each political party to compete in the Sept. 13 election.

A Nevada judge will hear arguments in the case May 19.

Democrats support Miller's interpretation of the untested law. They argue Republicans have no hope of winning in court because Miller, as the state's chief election officer, decides contest rules, not the political parties.

Washington lawyer Marc Elias is serving as the lead counsel for the Democrats. He has represented the Democratic Senatorial Campaign Committee and the Democratic Congressional Campaign Committee in the past.

"Nevada's law does not interfere with the party's internal processes for nomination or endorsement," the legal brief by the Democrats states. The Nevada Republican Party "is free to endorse its preferred candidate at the next state central committee meeting and to broadcast that endorsement via email, mail, telephone, canvassing, television, radio, or any other means at its disposal."

The election will fill a seat vacated by Republican Dean Heller, who moved from the House to the Senate on Monday to take the U.S. Senate post formerly held by Republican John Ensign.

Ensign left amid an ethics investigation into his extramarital affair with a former employee.

The Nevada GOP says it is reviewing the Democrats' motion and will file a response in court early next week.

"It's evident that national Democrats are so desperate to pick up a seat that they are calling in a Washington insider with close ties to the (Democratic Congressional Campaign Committee) to meddle in our state's affairs," said Mari Nakashima, GOP spokeswoman.

If the GOP prevails, it could spell trouble for tea party favorite Sharron Angle, who won the GOP Senate primary last year then lost to Democratic Majority Leader Harry Reid. She was the first candidate to enter the House contest before Heller resigned and the race was still focused on the 2012 election.

GOP Chairman Mark Amodei could be the party's favorite contender. Other Republicans who've said they will run include retired Navy Cmdr. Kirk Lippold and state Sen. Greg Brower.

State Treasurer Kate Marshall and fellow Democrats Jill Derby and Nancy Price, both former university regents, also announced plans to run in the special election.