Each campaign seems to believe that these highly local issues could swing one of the closest races in the nation in its favor.

The North Dakota Republican Party called on Heitkamp this week to return thousands in contributions from Weitz & Luxenberg, a law firm that has represented anti-fracking groups in the past, because it says the contributions indicate her hypocrisy on energy issues. The law firm is Heitkamp's second-largest donor, having contributed at least $22,400 to her campaign.

“Heitkamp has spent hundreds of thousands of dollars and several months desperately trying to distance herself from the disastrous Obama-Reid approach to oil and energy production, while simultaneously taking money from liberal extremists determined to shut down North Dakota's energy sector. Heitkamp's willingness to accept thousands in campaign cash from extreme anti-drilling, anti-fracking, New York City environmentalist trial attorneys shows North Dakota where her true allegiance lies," said North Dakota GOP Chairman Stan Stein. 

The National Republican Senatorial Committee also called for Heitkamp to return the donations on Wednesday. Heitkamp campaign spokesman Brandon Lorenz said in an email to The Hill that the candidate would not be returning the donations, and in response North Dakota GOP spokesperson Anna Nix said that Heitkamp should return the donations "from people who threaten the livelihood of North Dakotans" if she "wants to maintain credibility in the state."

Fracking — or hydraulic fracturing, in which a high pressure mixture of chemicals, sand and water is injected into rock formations to unlock natural gas — accounts for a large portion of jobs and energy production in the state, making it a popular practice that Heitkamp would have to support to back up her self-described independent credentials.

In tying Heitkamp to the standard-bearers of the Democratic Party in the White House and the Senate, the North Dakota GOP is working to paint the candidate as working in lockstep with her party — an attack which, if it sticks, could ultimately be fatal to her Senate bid. 

She's running in a red-leaning state where GOP presidential nominee Mitt Romney holds a comfortable lead, and her best chance at defeating Berg is to paint herself as centrist and capable of standing up to her party.

And that's exactly what she's done in recent appearances on the stump and ads. An ad released last week by the Heitkamp campaign touted her ability to stand up to President Obama on energy issues, including the Keystone oil pipeline, popular in North Dakota but a project that Obama has blocked, and cap-and-trade legislation, which could be a costly burden on some North Dakota energy plants.

But Heitkamp has also gone on the offense over the past week, launching an attack on what it's characterizing as Berg's time with Goldmark Property Management, a company with a history of violating fire codes. 

Buildings managed by Goldmark Property Management were cited in 2000 for missing smoke detector batteries, inoperable kitchen windows and inaccessible fire extinguishers, according to a report from the Fargo Forum. And Berg, during his time in the North Dakota state legislature, attempted to block legislation that would expand fire prevention rules and regulations.

Her campaign has issued multiple releases over the past week outlining the timeline of his involvement with the company, even as Berg insisted he left the company before it violated fire codes. The issue gained more traction when Heitkamp's campaign countered with documents on which Berg had listed Goldmark as his employer and had been cited as a company spokesman by news reports, though she has still been unable to produce legal documents that list him as owner of the company.

Nix pointed this out in her statement on Heitkamp's refusal to return funds to Weitz & Luxenberg, saying also that, to maintain credibility in the state, she'll have to produce legal documentation.

On Thursday, the North Dakota Democratic NPL Party released documents they say prove Goldmark overcharged the state Department of Veterans' Affairs over $6,000 in rent on their office in Bismarck. A representative from Goldmark disputed that the company had overcharged the VA, but the two parties came to an agreement by which the VA could give a 30-day notice on vacating its lease if it agreed to not pursue a refund of the alleged overpayment.

The North Dakota VA corresponded on the issue with Goldmark employee Kelly Zander, who was listed on financial disclosure forms as Berg's campaign treasurer.

The Party is calling for Berg to return the funds that the VA alleges Goldmark overcharged them.

“Rick Berg should stop trying to mislead North Dakotans about his involvement with Goldmark and refund the $6,543 that an employee and close Berg ally improperly withheld after overcharging the Veterans Administration," said Rania Batrice, spokeswoman for the Dem-NPL.

Berg's campaign did not dispute the new details from the ND Democratic-NPL, but did point to comments from Goldmark Property Management's CFO reported on a local news station that "since 1986, [Berg] has had zero involvement with Goldmark Property Management or the residential management business."

However, Berg has said that he was involved with Goldmark-Schlossman, which he claims has no "legal affiliation" with Goldmark. A 2011 real-estate license issued to Zanger identifies him as an employee of Goldmark-Schlossman, indicating the VA's dispute was with the company with which Berg was affiliated.

Berg campaign spokesman Chris Van Guilder dismissed the focus on Goldmark as a distraction from Heitkamp's support of Obama.

"Without any credible evidence to support her false claim, 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 continues to grasp at straws in a desperate attempt to distract from her record of supporting President Obama and his failed policies, including Obamacare. That’s what this election is about, and no amount of manipulated attacks change the facts," he said.

But Heitkamp's campaign used the North Dakota GOP's call to return funds to the anti-fracking law firm to refocus the conversation, yet again, on Goldmark. Campaign spokesman Lorenz said that the attention on the firm was just an attempt on Berg's part to distract from his involvement with the company.

"Rep. Berg is now launching desperate attacks to cover up his absurd lie last week that he had no involvement with Goldmark and its history of fire code violations, even though he founded the company and served as its spokesman," Lorenz said.

The Thursday developments launched by both parties on each issue likely mean that they won't be going away, as the campaign shifts into a more combative phase just seven weeks out from the election.

—This post was published at 12:08 p.m. and updated at 8:30 p.m.