A University of Delaware debate Wednesday night was mostly a civil affair among the four nominees for the U.S. Senate and House. But it turned contentious when Republican Senate challenger Rob Arlett brought up the fact that three-term incumbent Tom Carper slapped his first wife about four decades ago, when he was state treasurer.

It was midway through their hourlong debate when Arlett brought back the topic, which was an issue during Carper’s first race for Congress in 1982. Carper had admitted to hitting her during a deposition in a custody dispute involving his then-wife and her ex-husband.

Arlett began by asking Carper about a group that recently “stormed’’ Carper’s office in Washington, “demanding your resignation because of your past history of assault against a woman.”

Carper didn’t address striking his first wife during his initial response to Arlett. Instead he said it was just one “really bizarre” woman who showed up at the congressional office “looking for some kind of episode and she was disappointed when she left.” He also referred to the woman as “bonkers.”

Arlett, a Sussex County councilman who ran President Donald Trump’s Delaware campaign in 2016, persisted.

“I guess the ultimate question is you are on the record, it’s now public information, for abusing your former wife,” the challenger said. “That was a lie for 19 years about that. There was a big cover-up, there was big collusion.’’

Carper exploded.

“That is baloney,’’ he yelled. “Let’s set the record straight, my friend. Every one of us makes mistakes. God knows I made my share of mistakes. Forty years ago, I made a mistake. I owned it. I didn’t hide it. It was public knowledge.”

He said this then-wife Diane, who died several years ago, later defended him, and her son is now is “like my third son.” Carper has two grown sons with his current wife, Martha Carper.

“Every other year for 40 years, people like you, my friend, try to dredge this up,’’ he told Arlett, “to make mischief, political mischief, for me. It doesn’t work. It didn’t work 30 years ago. It didn’t work 20 years ago. It didn’t work 10 years ago. And you know what, it’s not going to work this time either.”

Finally, moderator Ralph Begleiter ended the exchange. “I’m going to going to call a halt to that,’’ he said as Carper continued and Arlett started to respond. “I’m going to ask a halt to that.”

Then Begleiter, a former CNN reporter and former director of UD’s Center for Political Communication, moved on to a question about social media abuses, and the debate resumed in a more civil manner.

During the rest of the discussion, attended by about 300 people at the university’s Mitchell Hall, Carper and Arlett calmly explained their differences.

For example, Carper rejected Brett Kavanaugh nomination to the U.S. Supreme Court. Arlett would have voted yes.

Carper supports Obamacare. Arlett wants government out of health care.

The incumbent wants tougher action on climate change. The challenger doesn’t.

Carper voted against Trump’s tax cut package. Arlett supports it.

On gun control, Arlett says the issue is not what weapons Americans can buy and shoot.

“Anybody who does harm to another human being, I believe, is mentally ill,’’ Arlett said. “Whether it be using a gun, using a vehicle or using a knife. We have rules we have to enforce that are not necessarily being enforced today. I also think we have to take a focus on mental care in this nation and stop playing around with that game.”

Carper responded by reading from the Constitution.

“It says, ‘A well-regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms shall not be infringed,’ ’’ Carper said. “Doesn’t say anything about assault weapons, doesn’t say anything about bump stocks. Doesn’t say anything about armor-piercing bullets.”

A UD poll shows Carper with a wide lead in Delaware, where Democrats hold a nearly 2-1 voter registration advantage over Republicans.

That same poll showed first-term U.S. Rep. Lisa Blunt Rochester with a comfortable lead over Republican Scott Walker.

Before Carper and Arlett took the stage, the House candidates revealed their own differences to the audience and those following on the radio and online.

One major distinction is their life experiences.

Blunt Rochester is the daughter of Ted Blunt, a former Wilmington education official and City Council president. She spent several years in top state government posts, including secretary of labor.

Walker, by contrast, is a 67-year-old self-identified alcoholic who has slept in his battered old Toyota station wagon in the last year and ekes out a living selling flowers to malls and shopping centers. He once rented homes to low-income tenants, but ran afoul of local building codes and regulations and has since given up his properties.

He doesn’t raise money and campaigns alone in the station wagon that is painted with his name. He paints his own nature-themed signs, nailing them to trees and planting them in high-traffic areas.

But Walker, who finished fifth in a six-person Democratic primary against Rochester in 2016, switched parties this year and surprised the GOP’s Lee Murphy by winning the primary in September.

But the state party has disavowed his candidacy, in part because of Facebook posts like the recent one in which he called Blunt Rochester an “Aunt Tom,’’ a twist on a historical racial slur.

Walker acknowledged that he was an unconventional candidate Wednesday. He described himself as a centrist, albeit one who supported Trump’s tax cuts, would close the Centers for Disease Control and Prevention and other federal agencies to stem the ballooning deficit, and doesn’t support more restrictions on firearms.

He said the right to bear arms was the Second Amendment because the nation’s founding fathers considered it “the second most important amendment’’ after the right to free speech.

“Gun ownership is the key to citizen safety,” he said, adding that there is no need for more regulations when “judges are not locking up people who commit crimes with guns.”

He said residents of Wilmington don’t want their rights restricted or taken away. And he said around his home in Sussex County, “Everybody says, ‘Don’t touch my gun.’ ”

Rochester, who describes herself as a progressive who aims to bridge partisan divides, countered that gun control is “probably one of the issues I’ve heard about the most” in schools and neighborhoods she has toured.

“There’s a lot of fear and angst,’’ she said, adding that most people favor “common sense approaches,’’ such as banning assault weapons.