Rep. Rick Berg (R) held a press conference in Minot, N.D., to highlight contributions Heitkamp received from the Council for a Livable World, according to a Minot Daily News report, an organization he characterized as anti-nuclear-weapon, anti-defense and a potential detriment to North Dakota's jobs.

"They support slashing our defense budget to the bare bones. They oppose missile defense technology, and if The Council for a Livable World had its way, they would abolish all nuclear weapons. It's troubling that while this community is engaged in hard work to ensure the survival of the Air Force base, Heidi HeitkampMary (Heidi) Kathryn HeitkampPompeo lacks votes for positive vote on panel Overnight Defense: House to begin work on defense policy bill | Panel to vote Monday on Pompeo | Trump to deliver Naval Academy commencement speech | Trump appeals decision blocking suspected combatant's transfer The Hill's Morning Report: Inside the Comey memos MORE has been soliciting funds from a group that wants to shut down military bases just like the one here in Minot," Berg said, according to the Daily News.

The organization, which bills itself as a "non-profit, non-partisan advocacy organization dedicated to reducing the danger of nuclear weapons and increasing national security" on its website, did contribute more than $12,000 to Heitkamp, making it her third-largest donor. Berg called for Heitkamp to return the funds.

And the North Dakota GOP focused in on Heitkamp's donations from employees of Weitz & Luxenberg, a law firm that represented anti-fracking groups in the past. The state party called for her to return the $22,400 she received from employees of the firm, calling it hypocritical of her to proclaim support for bipartisan energy policies but keep the funds.

The focus on Heitkamp's donations is reflective of the difficulty both candidates face in pinning down their opponents on policy issues in the state. Heitkamp has made an extra effort to frame herself as bipartisan, touting her support for Republican-supported proposals like the Keystone oil pipeline and criticizing the healthcare reform law.

And Republican policies aren't much of point of attack for Democrats in a state in which Mitt Romney has a double-digit lead on President Obama.

North Dakota's economy is doing better than the nation's as a whole, as well, making jobs and the economy a less appealing focus for candidates who, according to the most recent independent poll, released this week, are tied at 47 percent each.

Instead, both have focused on more personal issues, specific to the opponent, to try to shave off support. For Berg and Republicans, it's been Heitkamp's donors; for Heitkamp and Democrats, it's been Berg's involvement with the Goldmark family of real estate companies, one of which — Goldmark Property Management — violated fire codes and received numerous tenant complaints.

Heitkamp's campaign has worked to tie Berg to the company, though he says he had no involvement with GPM when it came under fire for poor management practices and that it has had no legal connection to any of Berg's other businesses, including another Goldmark firm he helped launch.

And in response to Berg's focus on her contributions from the Council for a Livable World, Heitkamp's spokesman Brandon Lorenz pointed out the $17,500 GPM contributed to Berg for his Senate run and called the new attack a distraction.

"Heidi believes in a strong national defense, and that means securing vulnerable nuclear weapons in unstable countries, but also preserving our missile defense systems and North Dakota bases," he said in a statement. "Rep. Berg is trying to distract from new polling showing he's been unable to win over North Dakotans with his flimsy denials and attempt to mislead the voters over his involvement with Goldmark Property Management, a company that has a C- rating from the Better Business Bureau. North Dakotans know that if Rep. Berg misleads us about his business here at home, he can't be trusted in Washington."