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

The former Maine governor is running as an independent for the vacancy created by Republican U.S. Sen. Olympia Snowe’s retirement. While celebrating the opening of his headquarters Monday, he said he expects the contest 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 the Maine contest. Democrats are fighting to hold a slim majority. Many believe King would ultimately side with Democrats.

He already has endorsed President Barack Obama’s re-election campaign. But King is refusing to say whether he’ll caucus with Democrats or Republicans if 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’ll side with Democrats. Maine Republicans have attacked King for his former wind power company’s loan guarantee. And the National Republican Senatorial Campaign already produced a negative web video suggesting King’s candidacy is the result 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.”

An early poll shows King has a fairly large lead in the U.S. Senate race, but that’s mostly due to name recognition with the race still in its infancy, the pollster says.