LONDONDERRY, N.H. — Since he became the Republican Senate candidate in New Hampshire, Scott Brown, a former United States senator from Massachusetts, has hammered one theme: that the Democratic incumbent, Jeanne Shaheen, has been a rubber stamp for President Obama, “voting with him 99 percent of the time.”

He mentions this several times a day. At a factory tour here recently, Mr. Brown was so eager to make the point that he upped the ante and blurted out to reporters that Ms. Shaheen voted with the president “over 100 percent of the time.”

It was a small but telling moment. Mr. Brown has gained traction by lashing Ms. Shaheen to the president, who won New Hampshire twice but whose job approval ratings here are lower than ever: 37 percent approve, while 60 percent disapprove, according to a poll released Thursday by the University of Massachusetts Lowell.

Mr. Brown’s tactics have brought him results. Though Ms. Shaheen, 67, is a well-liked, longtime fixture in state politics who was governor before she was elected to the Senate in 2008, Mr. Brown, 55, who has lived full time in the state for less than a year, is now tied with her in the polls. Most polls show Ms. Shaheen ahead by two or three percentage points, but the differences fall within the margins of sampling error, and analysts call the race a tossup. It is one of a handful that could determine which party controls the Senate next year.