The GOP presidential nominee appears in an ad for Berg pitching the candidate as the "51st senator to help reduce the size of government, repeal the government takeover of healthcare, and cut taxes on small business."

"[Sen.] John HoevenJohn HoevenMcCain absence adds to GOP agenda’s uncertainty McCain diagnosis looms over GOP healthcare talks This week: ObamaCare repeal faces latest setback in Senate MORE [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 FlakeJeff FlakeSenate defeats ObamaCare repeal measure Passing the DACA legislation will provide relief to children living in fear OPINION | Healthcare vote a political death wish for GOP in 2018 MORE, 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 HeitkampHeidi HeitkampSenate heading for late night ahead of ObamaCare repeal showdown Overnight Healthcare: Four GOP senators threaten to block 'skinny' repeal | Healthcare groups blast skinny repeal | GOP single-payer amendment fails in Senate GOP single-payer amendment fails in Senate MORE and Romney ahead of President Obama by 14 percentage points.

Obama is serving the same purpose for one Democratic Senate candidate, Rep. Chris MurphyChris MurphySenate heading for late night ahead of ObamaCare repeal showdown Senate releases 'skinny' ObamaCare repeal bill Dems don’t want to help GOP improve repeal bill MORE 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: