U.S. Rep. Keith Ellison offered a public apology Thursday after calling his opponent "a lowlife scumbag" during a contentious radio debate in which the Fifth Congressional District candidates traded insults and accusations.

During the debate on KFAI-FM, sparks flew when Chris Fields accused Ellison of paying a Washington, D.C.-based political research firm to dig up dirt on Fields, whose ex-wife filed for a restraining order against him in 2006 when they lived together in Southern California. Fields has since remarried.

Ellison called Fields stupid for discussing the domestic dispute, then denied that he had anything to do with the release of information.

"You're a scumbag," Ellison told Fields. "You're a lowlife scumbag."

In response, Fields began offering commentary on Ellison's child support payments and recent divorce.

Ellison, a three-term Democratic congressman, later apologized for the outburst in a public statement. Fields is his Republican challenger.

"I acted beneath my personal standard as a public official, and I apologize," Ellison said.

After the debate, Ellison and Fields agreed to remain civil in the future.

But the storm raged before the calm. The KFAI broadcast went silent mid-debate, cutting away during a particularly tense moment. But when the audio returned, the candidates continued to trade barbs.

Things escalated again as they debated the education and employment disparities between black and white residents in the Fifth District, which covers eastern Hennepin County, including Minneapolis. Fields has made the employment and education gaps a central theme of his campaign.

"I'm trying to remove the biggest obstacle to fixing things," Fields told Ellison. "You."

Fields' campaign released a statement after the debate, arguing that Ellison "lacks the right temperament" to serve in Congress.

Ellison and Fields had been scheduled to debate again next Tuesday at the Minneapolis Urban League, 2100 Plymouth Av. N. The moderator is now trying to arrange separate interviews with the candidates, Fields' spokeswoman said.

Corey Mitchell is a correspondent in the Star Tribune Washington Bureau. Twitter: @CMitchellStrib