The poll, conducted by Mason-Dixon Polling and Research for Valley News Live, shows Heitkamp leading U.S. Rep. Rick Berg (R) 47 percent to 46 percent in the competitive race. Heitkamp and Berg are competing for the seat being vacated by retiring Sen. Kent Conrad (D).
Two previous polls released by the Democratic Senatorial Campaign Committee in Washington also had Heitkamp with a lead in the race. According to data compiled by HuffPost Pollster, the two polls from earlier this year showed the Democrat with a five-point lead. The Mason-Dixon poll is the first independent poll in the race.
The Valley News Live poll shows Berg with a commanding lead over retired Naval officer Duane Sand in Tuesday's Republican primary.
Heitkamp, a former state attorney general and gubernatorial candidate, and Berg, a one-term congressman, are locked in a competitive race in the Republican-leaning state. North Dakota voters have a history of preferring Democratic congressional candidates, having sent an all-Democratic delegation to Congress from the early 1980s until 2010.
The race has become one of the highest-priority contests in the battle for control of the Senate, with the DSCC spending $76,000 in the group's first ad buy on behalf of a Senate candidate in April. The ad accused Berg of voting with Washington Republicans in Congress instead of in North Dakota interests. Heitkamp's campaign has made Berg's voting record, particularly on agricultural issues, a main theme of the race.
In April, a Republican businessman in North Dakota ran a statewide radio ad attacking Heitkamp's record and calling her "Heidi-Ho" in a move that drew criticism from Heitkamp's campaign.