Former North Dakota Attorney General Heidi Heitkamp (D) holds a six-point lead over Rep. Rick Berg (R-N.D.), according to a new poll conducted for the state's Democratic Party.

Heitkamp leads Berg by 50 to 44 percent, according to the poll. She's well-known in the state, with 56 percent of voters viewing her positively and 35 percent negatively, while Berg is underwater at 43 percent positive and 48 percent negative, according to the poll.

While partisan polls should always be taken with a grain of salt, this one might be fairly on-target: It also shows Mitt Romney leading President Obama by 54 to 35 percent, a mark that seems reasonable given that Obama took 45 percent in the state four years ago. This is the latest sign that Republicans should be nervous about what originally appeared to be a slam-dunk race in the heavily conservative state.

The poll suggests Berg's campaign ads, which so far have largely focused on tying Heitkamp to Obama with negative ads, might not be helping him: 45 percent of voters in the poll say Berg is running the more negative campaign, compared to 26 percent who say Heitkamp is.

The poll of 400 likely voters was conducted from July 24-26 by DFM Research and has a margin of error of plus or minus 4.9 percentage points.