NASHVILLE (AP) — A lawsuit backed by the Tennessee Democratic Party seeks to block new redistricting maps for the state House and Senate, arguing Republican lawmakers who drew the maps violated the state Constitution to keep a firm grip on their partisan advantages.

The challenge filed in Davidson County Chancery Court by three voters Wednesday is the first over state lawmakers' once-a-decade redistricting process. It claims Republicans, who hold supermajorities in both chambers, divided more counties than needed in the House to create districts with roughly equal populations, and numbered the Senate districts incorrectly. Republicans have argued their map-making work falls in line with state and federal requirements.

The challenge does not target the higher profile U.S. House map, which has drawn scrutiny for carving growing, left-leaning Nashville into three districts that extend far into suburbs and rural areas, favoring Republicans.

The move spurred Democratic U.S. Rep. Jim Cooper to declare he will not seek reelection because he could not win any of the three Nashville seats. This week, another Republican candidate lined up in a growing GOP field for the vacant seat — former Tennessee House Speaker Beth Harwell of Nashville.

The Tennessee Democratic Party said it is footing the bill in the lawsuit over the state districts.

"Tennesseans should pick their own representatives and not the other way around," Tennessee Democratic Party Chairman Hendrell Remus said in a news release. "From the very beginning, we doubted that the Tennessee redistricting process would be open and fair. Unfortunately, Republicans also violated the law while gerrymandering our state."

The House plan puts Democratic incumbents in the same district twice and Republican incumbents are paired once, not including lawmakers leaving office.

On the Democratic side, Reps. London Lamar and Torrey Harris would be in the same Memphis district and Reps. Gloria Johnson and Sam McKenzie would be in the same Knoxville district. Lamar is considering a run for an empty Senate seat. Johnson has said she will move and run for another House seat.

Meanwhile, GOP Reps. Jerry Sexton and Rick Eldridge would be in the same district that includes Grainger County and part of Hamblen County.

Democrats argue the map dilutes the power of minority voters, particularly in how it divides up Memphis and Rutherford County, which includes suburbs of Nashville, a city and region that boomed since the last census. The map splits 30 counties, the maximum permitted for the state House.

The lawsuit says a Democratic proposal would have divided up seven fewer counties and put the districts closer in line with the ideal population per seat.

On the Senate side, the lawsuit argues that Republican lawmakers did not number four districts that include Nashville consecutively, which matters because senators serve four-year terms and their elections are staggered. The lawsuit says the numbering used puts three of the Nashville seats on the ballot in gubernatorial years and one during the presidential years.

GOP Senate Speaker Randy McNally told reporters Thursday he thinks the state can successfully defend the maps.

On the congressional map, meanwhile, it remains unclear what kind of challenge might follow.

Any litigation faces significant hurdles. The U.S. Supreme Court has ruled that federal courts will not referee partisan gerrymandering.

Additionally, Nashville likely does not have enough minority voters to make up a district's majority, a key argument under federal voting rights protections. A recent U.S. Supreme Court ruling in Alabama further dampens Nashville's prospects in court, with justices deciding that elections were too imminent to consider changes to the state's congressional map.

Tennessee also lacks anti-gerrymandering laws for congressional maps similar to those in North Carolina and Ohio, where the states' supreme courts have rejected their maps.

The new, open Nashville seat has drawn several Republican candidates aside from Harwell.

Morgan Ortagus, the former the State Department spokesperson under former President Donald Trump, has Trump's endorsement in the race. Video producer Robby Starbuck has U.S. Sen. Rand Paul's endorsement. Retired National Guard Brig. Gen. Kurt Winstead has filed federal candidacy papers and small business owner Baxter Lee is running. Maury County Mayor Andy Ogles is considering a bid.

The Democratic side is less clear. Odessa Kelly, a community organizer from Nashville, had been running for Cooper's seat, and says she is now assessing what her next steps will be.a