The poll, while more than a month old, showed 44 percent of North Dakota voters likely to choose Berg, compared to 40 percent for a Democrat. It also showed Berg upside-down in voters' confidence that Berg shared their views, down 24-37 on that issue.

While North Dakota does not have voter registration, the state leans heavily Republican, and President Obama lost by eight points to Sen. John McCainJohn McCainBush biographer: Trump has moved the goalpost for civilized society White House to pressure McConnell on ObamaCare McCain: Trump needs to state difference between bigots and those fighting hate MORE (R-Ariz.) there in 2008.

A Republican strategist argued that since Gulleson had also been mentioned as a Senate candidate, her decision to run instead for the House reflects an admission by Democrats that Berg is unbeatable.

"On the same day [that Democrats are] trying to spin Rick Berg's supposed vulnerability, their top recruit — among an already weak bench — decides it's an easier path to run for the House, instead of against Berg for the Senate," the strategist said. "Sounds like she doesn't believe their memo either."

Berg arrived in Washington less than a year ago as part of a Republican wave that brought 63 GOP freshmen to the Capitol.

"Team North Dakota," as the three-member delegation of Democrats who represented the state together for 18 years called themselves — was disbanded in 2010 and became two-thirds Republican. Pomeroy lost his seat to Berg and former Gov. John HoevenJohn HoevenGOP senator criticizes EPA head's closed-door meeting in North Dakota Senate GOP eyes end to August session McCain absence adds to GOP agenda’s uncertainty MORE (R) won the seat left open by Dorgan's retirement.

With Conrad — North Dakota's sole remaining Democrat in Congress — set to retire, Republicans are hoping to finish the clean sweep they started in 2010.

"Contrary to Republican spin, Democrats fully expect a competitive Senate race in North Dakota next year," said Democratic Senatorial Campaign Committee spokesman Matt Canter.

The poll of 614 North Dakota voters was conducted Aug. 14-16 and had a margin of error of plus or minus four percentage points.

- This post was updated at 12:30 p.m.