NASHVILLE (AP) — The campaign arm for U.S. Senate Democrats endorsed James Mackler on Thursday for an open seat in Tennessee.

The Democratic Senatorial Campaign Committee praised Mackler's military service in its endorsement. Mackler is a former Army helicopter pilot and served in the Judge Advocate General Corps, where he was an Army prosecutor focused primarily on sexual assault and harassment cases. He is now an attorney in Nashville.

In the August primary election, Mackler faces Memphis environmentalist and activist Marquita Bradshaw and Nashville consultant and professor Diana Onyejiaka.

Republican Sen. Lamar Alexander is not seeking reelection to the seat in Tennessee, a state where Republicans have held both slots in the Senate since 1994.

"(Mackler) will work tirelessly on behalf of Tennesseans to protect pre-existing conditions coverage and health care access in rural communities, cut red tape to help Tennessee's small businesses, and take on the special interests that have corroded our democracy and hurt Tennessee families," Sen. Catherine Cortez Masto of Nevada, Democratic Senatorial Campaign Committee chairwoman, said in a statement. "In the Senate, James will be a voice for all Tennesseans, and we are proud to support him in this race."

Former U.S. Ambassador to Japan Bill Hagerty and trauma surgeon Manny Sethi are the top Republican contenders in this year's race.

The contest presents another steep climb for Tennessee Democrats, who recruited their best candidate in years in 2018 and still lost a Senate race by double digits.

Popular Democratic former Gov. Phil Bredesen vied to replace former Republican Sen. Bob Corker, who has retired. Mackler left that race after Bredesen decided to run; then Mackler endorsed him.

Bredesen lost to now-Republican Sen. Marsha Blackburn by more than 10 percentage points.

In this year's contest, Bredesen has endorsed Mackler.