"[Sen.] John Hoeven [R-N.D.] needs a conservative teammate to help stop the liberal Reid-Pelosi agenda," he says in the ad.
Romney has appeared in ads for Rep. Jeff Flake, running for Senate in Arizona; Rep. Denny Rehberg, running for Senate in Montana; and state Treasurer Richard Mourdock, running for Senate in Indiana. All of the ads frame the race as pivotal to the success of a wider Republican agenda.
North Dakota, Arizona, Indiana and Montana are all states where polls show a lead for Romney, but the Republican candidates are locked in tight battles and will need to solidify their base by Election Day to ensure a win.
The most recent independent poll of the North Dakota Senate and presidential races show Berg up by 2 percentage points over Democrat Heidi Heitkamp and Romney ahead of President Obama by 14 percentage points.
Obama is serving the same purpose for one Democratic Senate candidate, Rep. Chris Murphy in Connecticut. The president appeared in an ad for Murphy out last week — his first and only ad for a Senate candidate this cycle — vouching for Murphy on job creation and women's issues.
Most polls have shown Murphy ahead of Republican Linda McMahon in the Connecticut Senate race, and Obama has a double-digit lead there.
Watch Romney's ad for Berg: