BOSTON (CBS) – The U.S. Senate contest in New Hampshire is one of the biggest races to watch this year, and the outcome could change the balance of power in Washington.

A new WBZ/UMass-Amherst poll finds the battle between Jeanne Shaheen and Scott Brown is too close to call. Shaheen leads Brown among likely voters by just four points, 46% to 42%, well within the margin of error.

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Shaheen’s double digit lead melted away under the heat of a national case of Obama fatigue, the rise of some surprising issues, and the failure of gender-based political attacks that worked well against Scott Brown two years ago in his contest with Elizabeth Warren, but seem to have lost traction this time around.

“This race is tightening up,” says UMass pollster Ray La Raja. A key reason why, La Raja says, is Brown’s surprising competitiveness among female voters, a group Shaheen is counting on to break for her by a large margin.

But the WBZ/UMass-Amherst poll shows the gender gap at 11 points, barely half the margin Shaheen enjoyed in some earlier polls.

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Add in voters who say they’re leaning one way or the other and the gender gap shrinks to seven points, despite a recent barrage of anti-Brown attack ads geared toward women.

“I think she overreached here a little bit,” La Raja says. “And I think the voters realized it and it hurt her, because it’s become overly negative. Scott Brown is pro-choice so it didn’t seem so believable to some of these independent voters.”

“If you compare Massachusetts and New Hampshire, there are a lot more single women in Massachusetts and I think that might be driving some of this, single women are much more likely to be appealed to by that type of issue,” La Raja says.

Instead, unexpectedly, it’s issues like the war with ISIS, murderers of two men with close ties to New Hampshire and immigration questions raised by the southern border refugee crisis of last summer and the current Ebola scare that are vying with the economy for top billing among New Hampshire voters.

“Do we stay with what we know? Or, we’re angry, should we try someone new? That’s what the voters are thinking right now,” La Raja says.

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