The New Hampshire election is one of several close races that will determine whether Republicans win a Senate majority.

Scott Brown, 55, will contest this race for the third time in five years, following a pair of contests next door in Massachusetts in 2010 and 2012 in which he first won, then lost, the seat once held by the late Democratic Sen. Edward M. Kennedy.

Shaheen, 67, is a known political commodity in New Hampshire, elected to the Legislature, then three terms as governor as well as one in the Senate.

She also lost a race for the Senate in the strong Republican year of 2002 to John Sununu, then defeated him in 2008 when President Obama won the state.

“She’s won in a wave and lost in a wave,” says former Rep. Frank Guinta, a Republican running for his former seat. “I think she’s as susceptible as anyone is.”

Shaheen is supported by The League of Conservation Voters, the Democratic Senatorial Campaign Committee and the Senate Majority PAC, which is run by allies of Majority Leader Harry Reid, D-Nev., who have come up with ads attacking Brown.

Brown’s presence is explained by the view that even a well-established politician can lose if enough voters sour on Obama after six years.

Brown runs against the current occupant of the White House, who is plagued with low approval ratings. Shaheen votes “99 percent of the time with President Obama,” he says frequently. “Do you know how much she actually votes for you? Zero,” he told one audience.

Campaigning last week in Nashua at a company that makes prototype metal products, Brown stressed the importance of renewing the state’s economy, easing federal regulations and reducing energy costs. He favors repealing the nation’s health care law, even before any replacement is ready.

Two weeks ago, his office did not respond when asked if he would have voted for legislation that cleared Congress with Shaheen’s support to arm and fund Syrian rebels fighting Islamic State militants.