The ad, from the Democratic Senatorial Campaign Committee (DSCC), charges that Berg attempted to block laws that would've expanded fire-safety rules during his time in the State Legislature. It says that that effort was due to the fact that "a company he was associated with" violated fire codes.

"More evidence we just can't trust Rick Berg," a narrator says at the end of the ad.

Berg's ties to the company cited in the ad, Goldmark Property Management, have long been a focus of Democratic attacks. Though Berg founded the precursor to GPM, and has worked with other Goldmark companies in the past, he claims that during the period when GPM was cited for fire code violations, he wasn't involved in the company at all.

The DSCC ad, though, doesn't mention GPM by name, and gives little background on Berg's association with the company. It offers only visual clues — documents that Democrats believe tie Berg to the company, and which have appeared in ads before — to provide context, an indication Democrats believe their previous ads have taken hold to the point that simply a nod to the situation is enough to spark an understanding among voters.

The Goldmark issue has become central to an unexpectedly fierce campaign, as Berg has faced a strong challenge from Democrat Heidi HeitkampMary (Heidi) Kathryn HeitkampHouse passes bill to ease menu labeling rules under ObamaCare In 2018, Trump must be the small-business champion he claimed to be GOP goes on offense with 20-week abortion vote MORE for the state's open Senate seat.

The two most recent polls of the race, taken over the past 10 days, have given Berg a 10- and a 5-percentage-point lead over Heitkamp, and Mitt Romney holds a consistent double-digit lead over President Obama there. But Democrats believe they could pull out a win in the right-leaning state, as North Dakotans are known for splitting their tickets and Heitkamp is one of the party's top recruits.

Watch the ad: