There is a gender gap in the poll, with Berg getting 57 percent support from male voters to Heitkamp's 35 percent. The Democrat led among female votes, but by a much smaller margin: 45 percent prefer her while 42 percent support Berg. A Mason-Dixon poll conducted in early October showed a tied race, with both candidates pulling in 47 percent support. The North Dakota Senate race has always been considered uphill for Democrats, given the state's red tilt and Mitt Romney's advantage over President Obama -- but Heitkamp is also a strong recruit for the Democrats, and poll after poll has indicated a tight contest. In fact, the poll indicates that just 19 percent of likely voters identify as Democrats, far fewer than the 35 percent who say they are Republicans and the 42 percent who are independents. The memorandum states that, "Although the polling was conducted through random digit dialing, the poll skewed toward Independents and Republicans." For their part, the Heitkamp campaign criticized the poll for showing "a sample with a partisan makeup completely at odds with North Dakota exit polling and historical trends." Democrats made up 28 percent of the electorate in 2008, according to exit polls, while 38 percent of voters said they were Republicans. North Dakota does not have partisan voter registration. The poll was conducted Oct. 12-15, and surveyed 500 likely voters. It has a margin of error of plus or minus 4.4 percent. Updated at 11:50 with Heitkamp campaign statement. Polling editor Steven Shepard contributed to this post.