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 Henry HoevenThe Hill's Whip List: Republicans try again on ObamaCare repeal Air Force One is Trump’s new boardroom Overnight Finance: Trump strikes debt, spending deal with Dems | Deal shocks GOP | Fed’s No. 2 to resign | Trump keeps tax squeeze on red state Dems | House aims to pass budget next week 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 FlakeJeffrey (Jeff) Lane FlakeAuthorizing military force is necessary, but insufficient Republicans jockey for position on immigration McCain, Flake warn against 'politically-motivated penalties' for Canadian defense firm 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 HeitkampMary (Heidi) Kathryn HeitkampWells Fargo chief defends bank's progress in tense Senate hearing Dems plan to make gun control an issue in Nevada Red-state Dems need more from Trump before tax embrace MORE and Romney ahead of President Obama by 14 percentage points.

Obama is serving the same purpose for one Democratic Senate candidate, Rep. Chris MurphyChristopher (Chris) Scott MurphyChris Murphy’s profile rises with gun tragedies Murphy faces criticism from GOP challenger over fundraising email Democrat: Republicans who believe in more gun control afraid of being 'politically punished' 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: