Democrat James Mackler, a Nashville attorney and Iraq War veteran, is dropping out of next year's US Senate race in Tennessee after his bid became a long-shot following the entry of former Democratic Gov. Phil Bredesen.

Mackler made the announcement in a statement late Wednesday, saying he is "stepping back" from the race to replace Sen. Bob Corker, R-Tennessee, with plans to stay involved through a new political action committee that will support federal candidates.

His move comes one week after Bredesen, a former two-term governor of Tennessee from 2003 to 2011, entered the Senate race, leaving Mackler a sizable underdog to win the party's nomination in August.

"While I am stepping back from this U.S. Senate race, I will continue holding anti-service extremists like Marsha Blackburn accountable through a political action committee called 'Believe in Service,' Mackler said.

MORE: Bredesen hopes to bring Republicans and Democrats together

"It has been an honor to have been a candidate during this critical time in our democracy and I am humbled to have earned the support of so many across Tennessee."

Mackler, a newcomer to politics who quit his job from a Nashville law firm to launch his uphill Senate run in April, had impressed on the campaign trail and raised more than $750,000 along the way.

But Bredesen — a political moderate who was courted by Democratic leadership in Washington to run — is widely seen by Democrats as their best shot to put Republican-heavy Tennessee in play.

Mackler said his new PAC will support federal candidates who agree to protect and expand national service programs

His exit allows Bredesen, now the lone declared Democrat, the ability to pivot to the general election in November. Declared Republican candidates are U.S. Rep. Marsha Blackburn, R-Tennessee, former Rep. Stephen Fincher, R-Tennessee, and perennial candidate Larry Crim of Nashville.

Bredesen told The USA Today-Tennessee network he spoke to Mackler last week before deciding to enter the campaign after seven years away from elected office.

“I’ve gotten to know James Mackler over the past few months, including through several conversations in recent weeks about the Senate race," Bredesen said in a statement. "I admire his deep commitment to service to our nation.

"His ‘Believe in Service’ initiative is a great way to protect and expand critical national service programs. I look forward to watching the success of this initiative and working with James in the future.”

This is Joey Garrison 615-259-8236, jgarrison@tennessean.com and on Twiter @joeygarrison.