PORTLAND (AP) — The three leading candidates in the race to fill Sen. Olympia Snowe's soon-to-be-empty seat clashed Tuesday over outside money pouring into the campaign and the necessity of a tax increase in a spirited debate before business leaders.

Independent Angus King suggested that Republican Charlie Summers allowed the floodgates to be opened to more than $4 million in outside spending when he rejected King's pledge to try to limit spending by so-called super PACs. Summers, meanwhile, accused King, a former Maine governor, of reneging on his pledge not to run negative ads.

"Gov. King pledged to the state of Maine that he would not run one negative ad during this campaign. Yet he's the only one of the three of us standing up here running negative ads right now. I think people are tired of politicians who'll say one thing and do another," Summers said.

King said pointing out policy differences is different from the U.S. Chamber of Commerce attack ads that called him "king of mismanagement" and "king of spending."

"A negative ad is when they put your picture on the scene with this dark photo and a crown on your head. ... That's a negative ad," King said.

Sponsored by the Portland Regional Chamber of Commerce, the debate marked the first time in which King, Summers and Democrat Cynthia Dill faced off since Sept. 17.

It underscored some of the candidates' differences.

On campaign spending, King and Dill both support a constitutional amendment to overturn the Supreme Court's Citizens United decision that opened the door to unlimited spending by outside groups, while Summers does not.

On taxes, Summers signed Grover Norquist's no-tax pledge, while King and Dill acknowledged that some taxes will need to be raised to help balance the budget and pay down the nation's debt, now at $16 trillion.

Summers insisted that the path to economic prosperity calls for cutting government spending and creating a regulatory environment that allows businesses to grow.

"You should not be raising taxes on anyone in this economy," Summers said. "That's the quickest way to send this economy down the line."

King, who previously said he'd like to raise the capital gains tax, said he doesn't like raising taxes but thinks it will be necessary. He also took a jab at Summers: "Your pledge ought to be to the people of Maine — not to some guy in Washington."

Rejecting the use of the word "crisis," Dill said the current budget problems were created by tax cuts under President George W. Bush, wars in Afghanistan and Iraq and the collapse of Wall Street. She also repeated her concerns about growing income disparity and said her vision of the economy involves continuing "along the path that President Barack Obama got us started on to recovery."

She also said the negativity of politics and outside spending in the race have led to a growing sense of pessimism that she thinks can be addressed with a new generation of leaders. She vowed to work to restore "hope and optimism in Maine people in government and Maine leaders to solve problems."