Sanders is 9 percentage points ahead of Clinton in New Hampshire with 41 percent of the vote to her 32 percent. Vice President Joe Biden, who has not announced a run, places third with 16 percent.
Without Biden, the poll shows Sanders still maintaining a lead with 49 percent to Clinton's 38 percent.
In Iowa, Clinton has lost some ground and now leads by 11 points. The former secretary of state takes 38 percent of the vote to Sanders' 27 percent; Biden carries 20 percent. In a July NBC/Marist poll, Clinton's lead was more than double Sanders'.
Without Biden, the difference between Sanders and Clinton remains the same, with 11 points separating the two.
This is the third poll within one month that shows Sanders leading in the Granite State. A survey conducted in late August by Democratic firm Public Policy Polling had Sanders leading the democratic field by 7 points. Similarly, an August Franklin Pierce/Boston Herald poll also had Sanders ahead by 7 points.
In Iowa, other polls conducted show greater variation in Clinton's lead. A Des Moines Register/Bloomberg Iowa poll conducted in late August shows Clinton ahead by 7 points, while a Suffolk University/USA Today poll conducted in an overlapping time period shows her leading by 34 points.
These variations underscore the unreliable nature of early polling, which often surveys only a small fragment of eventual voters and is a poor predictor of the eventual outcome.
Nationally, Sanders continues to trail Clinton. According to the National HuffPost Pollster chart, which aggregates all publicly available national polls, Sanders is 22 points behind Clinton, but that margin has narrowed in August.
The NBC/Marist New Hampshire poll surveyed 996 registered voters with a subsample of 356 potential Democratic primary voters reached using live interviews via landlines and cell phones Aug. 26 through Sept. 2.
The NBC/Marist Iowa poll surveyed 998 registered voters with a subsample of 345 potential Democratic caucus goers via live interviews on landlines and cell phones Aug. 26 through Sept. 2.
Clarification: Language has been amended to describe more precisely the results of the poll in the absence of Biden.