(CNN) - A new poll in New Hampshire shows the number of likely voters there unsure of who will receive their presidential vote in November has spiked in recent weeks while the race remains locked in a statistical tie.
The percentage of likely voters who were undecided or favoring a non-major candidate there had remained steady between 6% and 9% since February 2011, according to the WMUR Granite State Poll, but that number shot up to 15% in the most recent sampling.
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This survey includes voters contacted between September 4 and 10, so it includes voters contacted both during and after the Democratic National Convention, which was taking place that week. All voters were contacted after the Republican National Convention, which was held the preceding week.
President Barack Obama and Republican presidential nominee Mitt Romney are statistically tied, with 45% favoring Obama and 40% supporting Romney. The poll's sampling error is plus or minus four percentage points.
The August iteration of this survey also found a tied race - 49% for Obama and 45% for Romney - with each candidate apparently having contributed nearly evenly to the growth in undetermined voters since then.
New Hampshire has four electoral votes and is one of the nine states considered a toss up on the CNN Electoral Map.
Independent likely voters in the Granite State were most likely to be undecided, with one in four fitting that category.
The survey showed Obama's favorability rating went up nine points to 52% since the August poll, while Romney's dropped three points to 45%. It also found Vice President Joe Biden and Republican vice presidential candidate Paul Ryan both had a favorability rating of 39%.
A separate poll of voters in Michigan also released Thursday found 16% of likely voters there were undecided. Michigan is rated as leaning in Obama's favor on the CNN Electoral Map, and the EPIC/MRA poll found he held a 10 point lead over Romney there.
Several other state polls released Thursday showed a statistically tied race in Colorado, Florida, Ohio, and Virginia. The NBC News/Wall Street Journal/Marist poll found Obama slightly ahead in Ohio.
The WMUR Granite State Poll was conducted by the University of New Hampshire and included 592 likely voters reached by telephone between September 4 and 10. It has a sampling error of plus or minus four points, while a larger sample of 635 registered voters had a sampling error of plus or minus 3.9 points.