BISMARCK, N.D. — Dwight Thompson is a solid conservative businessman who will be voting for a Republican for president and a Democrat for Senate.

He's a ticket-splitter, a rare political breed critical to Democratic Senate candidates in states like North Dakota, where Republican Mitt Romney is expected to easily win the presidential race while down-ballot contests remain stubbornly close.

North Dakota is enjoying an energy development boom and, unlike most of the rest of the nation, its economy is robust. Unemployment is 3 percent, the lowest of any state in the nation. The Senate race here largely has come down to the personalities of Democrat Heidi Heitkamp and Republican Rep. Rick Berg.

Thompson, 57, of Grand Forks, sees no contradiction in his decision. For him, it's about familiarity.

"There's just a kind of feistiness about (Heitkamp). I think that draws people to her," said Thompson, the chief financial officer of Altru Health System. "It's more for Heidi than against Berg."

Less than three weeks from Election Day, the North Dakota Senate race looms large in the broader race for control of the Senate.

Republicans need to gain four seats to take control of the Senate if President Barack Obama is re-elected, three if Mitt Romney wins the White House. That's because the vice president, who also serves as president of the Senate, has a vote in case of a tie.

The GOP task of gaining four has become increasingly difficult because of candidates like Heitkamp in races the GOP had once considered plum pickup opportunities.

Republicans romped in North Dakota in 2010. Berg took the state's lone seat in the House away from a Democrat as GOP Gov. John Hoeven easily won a Senate seat formerly in Democratic hands. Afterward, four-term Democratic Sen. Kent Conrad decided against running again.

Voters like Thompson have long since made up their minds about the presidential race. Romney, like every Republican presidential candidate in the state since 1964, is expected to win in a waltz.

But the contest between Berg and Heitkamp has remained close. A recent Mason-Dixon poll of the state conducted for Valley News Live had the two candidates tied at 47 percent each. The same poll showed Romney easily beating Obama and Republican Kevin Cramer comfortably ahead of his Democratic challenger, Pam Gulleson, in the race for Berg's House seat.

Ticket-splitting has a long history in North Dakota. Until 2010, the state's congressional delegation had been entirely Democratic since 1986, even as the state regularly voted for the GOP presidential ticket.

Republicans had hoped that the Senate race would be over by now, that Romney voters intrigued by Heitkamp would settle in with Berg. The first-term congressman's case to voters has been similar to Romney's: a healthy emphasis on the national debt, a dismissal of Obama's policies and the promise that he can bring his skills as a businessman to the Senate.

For much of the campaign, Berg has made a simple partisan appeal against Heitkamp, a former state attorney general. His ads and his stump speeches focused largely on her past support of Obama's policies, particularly the president's health care law. He said she would be a rubber stamp for Obama and Sen. Majority Leader Harry Reid. Berg's more recent ads have questioned Heitkamp's self-styled independence.

"Independent means you stand up to Obamacare," a narrator says in his newest one.

Berg says he wants to bring North Dakota's principles and its prosperity to the Senate. He's campaigned recently with Hoeven, a widely popular figure in the state.

Republicans recently have highlighted Heitkamp's donations from attorneys who oppose the drilling method known as hydraulic fracturing, or fracking, a big issue in this state where a booming oil industry has largely shielded people from the country's larger economic woes. Berg's supporters say such things show Heitkamp's independence is more style than substance.

"She won't represent North Dakota interests as well as Berg can," said Gene Nicholas, a retired state legislator and small grain and livestock farmer. "He's more mainstream."

Speaking while riding a tractor through his soybean field, Nicholas said Berg would also provide an important counterweight if Obama wins re-election.

"We've got trillion-dollar deficits and he's done nothing to solve that problem," he said. "Heidi won't help that."

Heitkamp has made preserving her independent image a centerpiece of her campaign.

In one television ad, Heitkamp, wearing a baseball pullover, helmet and batting glove on her left hand, hits line drives in a batting cage while her own narration describes how she expected political opponents to "try and hit me with all sorts of stuff" in a Senate campaign.

"I'm just getting warmed up," she says, winking at the camera.

And occasionally it has seemed as if she's running in opposition to Obama. At a North Dakota Chamber of Commerce forum earlier this month, she said the first thing she would tell Obama about his energy policy is: "You're wrong. You're wrong on energy. ... You made bad decisions."

"It's time for you to get into the real world," Heitkamp said she would tell the president.

Republicans have called this bluster, but it appeals to voters like Bob Rost, the Grand Forks County sheriff.

"I've known Heidi for a lot of years," Rost said. "I knew her when she was attorney general. There was a lot of issues that came up and she was not afraid to roll her sleeves up and deal with those sort of issues."

Even Berg's supporters say Heitkamp can be charming. Dennis Johnsrud, an Epping farmer who supports Berg, says the Republican must overcome Heitkamp's "likability factor."

"I think it's very effective," Johnsrud said of Berg's strategy of tying Heitkamp to the president. "I think it's the only reason (the race) is close."

Jackson reported from Washington.