New Hampshire Republican Senate candidate Scott Brown said man-made climate change has not been proven, a stance that an environmental group said is a flip-flop.

Brown, who represented Massachusetts in the Senate from 2010 to 2013, made the comment Saturday at a Republican primary debate. Former New Hampshire Sen. Bob Smith (R) agreed with him.

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NextGen Climate, backed by billionaire climate activist Tom Steyer, accused Brown of reversing his stance from the 2012 election, when Sen. Elizabeth Warren (D-Mass.) successfully unseated him.

“I absolutely believe that climate change is real and I believe there's a combination between man-made and natural,” Brown said in a debate at the time.

He continued: “that being said, we need to do everything; we need to work together, finding that balance to not only address our climate change problems but also to allow people to work and create jobs.”

NextGen, which started running advertisements against Brown last week, said the change shows that Brown does not care about New Hampshire’s interests.

“New Hampshire voters see Scott Brown for what he is: someone more interested in his own political career than in the issues that matter to Granite State voters,” said Pete Kavanaugh, the New Hampshire director for NextGen, in a statement.

“From healthcare reform to immigration to a woman’s right to choose and now to climate change, Scott Brown can’t make up his mind about what he believes.”

Brown is the front-runner to challenge Sen. Jeanne Shaheen Cynthia (Jeanne) Jeanne ShaheenSenate Democrats introduce bill to sanction Russians over Taliban bounties Trump-backed candidate wins NH GOP Senate primary to take on Shaheen Democratic senator urges Trump to respond to Russian aggression MORE (D-N.H.) in this year’s midterm election.