The ad in question highlights the business practices of a company called Goldmark Property Management, which the ad says is Berg's. Goldmark, according to the ad, was sued for withholding safety deposits and cited for ignoring fire safety codes, and the ad says Berg broke the lease of a deployed soldier.

"Rick Berg: treating seniors the same way he treats his tenants — like they're not worth anything," says a voiceover at the end of the ad.

But Berg is contesting the ad, claiming that he founded the precursor to Goldmark — called Midwest Management Co. — but left the company in 1987, seven years before it changed its name. According to Berg's account, he founded another real estate company in 1996, which had no legal or business affiliation with Goldmark Property Management.

On the basis of these claims, Berg asked that the ad be taken down. His campaign also slammed Heitkamp for what it's calling the personal nature of this particular attack, highlighting statements that Heitkamp had made previously condemning personal attacks in politics. The Democratic candidate has largely avoided attacking Berg's business history up until now.

However, the Heitkamp campaign issued a counter-fact-check asserting that its ad was correct, offering as evidence a screenshot of the Goldmark website from last month that lists Berg as the senior vice president of Goldmark Schlossman Commercial Real Estate Services — a fact that the Berg campaign did not dispute, saying instead in its fact-check that GSC Real Estate Services is independent of Goldmark Property Management.

The Heitkamp campaign also cited news reports in which Berg was quoted as a spokesman for Goldmark Property Management and said to have "cross-ownership" in all of the Goldmark companies, including Goldmark Property Management. In addition, Berg listed his employer as Goldmark Property Management on a campaign contribution form.

The candidates are likely to continue to debate the issue, but the back-and-forth does indicate a potential shift to a more combative stage of the campaign as the general-election race heats up in one of the tightest contests in the nation.