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Republicans are suddenly looking at a bonus opportunity to build a Senate majority amid signs that Scott Brown may be bringing his race for a Democrat-held seat in New Hampshire into toss-up territory.

Incumbent Democratic Sen. Jeanne Shaheen has long maintained a healthy lead in that race, often by double digits, but a recent poll showed Brown -- a former GOP senator himself -- closing the gap significantly.

"We knew the race was going to tighten, but nobody expected it to tighten this much," said Jeanne Zaino, political science professor at New York's Iona College.

Republicans need to win a net total of six seats to retake the Democrat-controlled upper chamber. Most political analysts say the GOP will almost certainly win three Democrat-held seats in November, but roughly a half-dozen others in Republican sights are more difficult to predict.

Brown’s recent surge, if it holds, gives Republicans one additional chance -- as well as a rare shot at a pick-up in New England.

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The latest WMUR Granite State Poll, released Aug. 21, shows Shaheen leading Brown 46-44 percent, with a margin of error of 3.4 percentage points.

Brown, a former Massachusetts senator, trailed by 12 points in a July poll released by the same outfit and conducted by the University of New Hampshire Survey Center.

Brown remains popular in Republican circles in part simply for winning a Senate seat in 2010 in heavily Democratic Massachusetts and for his reputation while on Capitol Hill as a principled voter. He’s campaigning again in the same trademark jalopy pickup he rode to victory in the 2010 special election.

However, Brown, who lost in his 2012 re-election bid, struggled early in this election cycle amid criticism he was a carpetbagger for moving to his New Hampshire vacation home to enter the race.

Shaheen is a former New Hampshire state senator and three-term governor who remains widely popular -- but she faces several challenges.

Like most Democratic incumbents, she is automatically tied to an unpopular president, fellow Democrat Barack Obama.

The situation has energized Republicans nationwide and is a major part of the Brown campaign strategy -- trying to connect Shaheen to such Obama administration headaches as the IRS scandal and the recent illegal immigration crisis at the southern U.S. border.

“Because of President Obama and Senator Shaheen, we have an immigration crisis on our hands," Brown campaign manager Colin Reed recently said. "Rather than fix the problem, she has made it worse by opposing efforts to secure the border and by supporting amnesty for young illegal immigrants.”

Shaheen also faces the historical trend of the party controlling the White House doing poorly in midterm or so-called “off-year” elections.

She has attempted to offset such disadvantages by trying to tie Brown, who has the backing of 2012 GOP presidential nominee Mitt Romney and Arizona GOP Sen. John McCain, to the Washington establishment, deep-pocketed political action committees and big business.

“This latest ad buy and the announced plans of Big Oil, Wall Street and other corporate interests supporting Scott Brown, will bring the amount of money they’re pouring into New Hampshire attacking Jeanne Shaheen to more than $7.3 million,” her campaign said. “These groups have already outspent the opposition by nearly 2-to-1.”

While the race appears close, University of New Hampshire politics professor Andrew Smith, who co-conducted the Granite State poll, says it remains wide open because most voters won’t become engaged or pick a candidate until after the Sept. 9 primaries and their children are in school.

Brown faces former New Hampshire Sen. Bob Smith and former state Sen. Jim Rubens in the primary. Shaheen, a first-term senator, is running unopposed.

Smith also provided data that disputes the narrative that 47 percent of New Hampshire voters are independents who can swing a race. Smith points to state records that show they are, in fact, registered “undecided” voters and their ranks are really closer to 42 percent.

“It’s a myth,” he said. “They’re just not publicly identified.”

Moreover, he points out, when voters are asked about the party to which they most identify, their answers show essentially an even split among Democrats, Republicans and independents.

Republicans, meanwhile, are expected to win the Democratic-held seats in Montana, South Dakota and West Virginia. Whether they can push to six, or beyond, is the big question.

The party has yet to lock down races in Arkansas, Alaska, Colorado, Iowa, Louisiana and North Carolina -- where Democratic incumbents were once considered to be in deep trouble, which makes the New Hampshire race a bonus opportunity.

An average of polls by the nonpartisan website RealClearPolitics.com shows that no candidate in those races has more than a 3-point lead, except in Alaska, where Democratic incumbent Sen. Mark Begich is now up by nearly 5 points.