BRUNSWICK — Already under attack from Republicans, independent U.S. Senate candidate Angus King is promising to run a positive campaign.

The former Maine governor is running for the vacancy created by Republican U.S. Sen. Olympia Snowe’s retirement. While celebrating the opening of his campaign headquarters Monday, he said he expects the race to get nasty.

“There’s going to be a lot of negative stuff — not coming from me,” he said. “No negative ads coming from this campaign whatsoever.”

The balance of power in the U.S. Senate could hinge on Maine’s contest. Democrats are fighting to hold a slim majority. Many believe that King would ultimately side with Democrats.

He already has endorsed President Obama’s re-election campaign. But King is refusing to say whether he will caucus with Democrats or Republicans if he is elected.

“I don’t want to caucus with anybody if I don’t have to,” he said. “That’s a decision that will be made after we go down there.”

Republicans are betting that he will side with Democrats. Maine Republicans have attacked King for his former wind power company’s loan guarantee. And the National Republican Senatorial Campaign has produced a negative Web video suggesting that King’s candidacy is the product of a “smoke-filled backroom deal.”

King is trying to take the high road. He’s calling for more civility in national politics.

“It’s about meeting in the middle,” he said. “And yes, it’s about compromise.”