Berg had been widely expected to win the race, and Republicans had counted on his victory in their failed attempt to take control of the Senate.

Heidi Heitkamp is a longtime Democrat, but her U.S. Senate campaign emphasized her political independence and personal charm. She posed in front of a passing train of tanker cars to advocate for a new oil refinery and swung a baseball bat to underline her disdain for claims that she'd be a loyalist to Democratic President Barack Obama.

It paid off Wednesday, when her opponent, Republican Rep. Rick Berg, conceded that Heitkamp's 3,000-vote margin was enough to give her a victory and said he would forego a recount.

Republicans and Democrats said Heitkamp's personal popularity, built during six campaigns and a long public career as tax commissioner and attorney general, had carried her to victory when other Democrats running for statewide offices lost by big margins.

"Anybody that underestimated Heidi Heitkamp's notoriety hasn't been paying attention," said Republican Kevin Cramer, who was elected Tuesday to replace Berg in the U.S. House. "There are not a lot of mysteries about Heidi ... She is a gregarious person."

Berg had been widely expected to win the race, and Republicans had counted on his victory in their failed attempt to take control of the Senate.

But Bismarck attorney Tom Dickson said he got a glimpse of the outcome a few days ago, when a state trooper handed him a speeding ticket. Dickson's sport utility vehicle is plastered with Heitkamp stickers, and the officer made a point of telling Dickson he had voted for her.

"I've been involved in a lot of campaigns," said Dickson, a former North Dakota Democratic chairman. The charismatic Heitkamp, he said, "is probably the single best candidate I have ever seen in my life."

Heitkamp appealed to voters in heavily Republican North Dakota by emphasizing her independence from her party. She has been harshly critical of the president's energy policy, is pro-oil and supports gun rights. Despite the Democratic label, many viewed the 57-year-old as more of an independent than a Democrat.

Allen Marler, 74, a retired farmer who lives in Rogers, said Wednesday that he voted for Berg but he thought Heitkamp would do a good job. In fact, he added, Heitkamp might have had a shot to become governor in 2000 if her campaign hadn't been sidelined as she battled breast cancer. She later recovered.

"I usually vote Republican, so I voted for Berg, but really, I was undecided," Marler said. "Heidi will do a good job. She's very energetic and she'll do a good job because she's already done good things for North Dakota before."

Heitkamp issued a statement Wednesday praising Berg and promising to work hard as a senator, but campaign workers said she wasn't available for further comment. Her campaign office was locked, and supporters attending a celebration at her Mandan home said she wasn't there. A spokeswoman said Heitkamp planned several public appearances Thursday.

The campaign was the most expensive Senate race in state history, with the candidates spending more than $8 million combined. Outside groups poured in millions more for television advertising and mailings on behalf of both candidates.

While voters like Marler criticized the negative tone of the race, Berg said he would not change anything about his campaign.

"Looking back at it, I feel like we left it all out on the field," he said.

Heitkamp is the first woman elected to represent North Dakota in either the U.S. Senate or House. Jocelyn Burdick briefly served in the Senate after her husband, Democrat Quentin Burdick, died in September 1992, but she was appointed by then-Democratic Gov. George Sinner.

Heitkamp's victory is the capstone of a political career that began in 1984, when she began the first of six statewide campaigns with an unsuccessful run for state auditor. She served as tax commissioner and attorney general before her unsuccessful run for governor.

She later led two successful ballot initiatives, one requiring greater state spending to fight tobacco use and another making it more difficult for governments to forcibly acquire property for economic development projects.

Berg was elected to the U.S. House two years ago in a wave of Republican victories. Five months later, he began campaigning for the Senate in an attempt to become the first North Dakota congressman to make the jump after just one term since Democrat Quentin Burdick in 1960.