The top three candidates for Maine's U.S. Senate seat clashed today over a number of issues in a debate in Portland.

PORTLAND, Maine (AP) _ Republican Charlie Summers and independent Angus King are clashing over the role of outside spending in the U.S. Senate race in Maine.

King suggested today during a debate in Portland that Summers and Republicans are to blame for negative ads by outside groups because Summers rejected a pledge to try to limit spending by so-called super PACs. Summers, meanwhile, accused King of reneging on his pledge not run negative ads.

King and Democratic candidate Cynthia Dill would support a constitutional amendment to overturn the U.S. Supreme Court's Citizens United decision that opened the door to unlimited spending by outside groups.

So far, more than $4 million's been spent by outside sources in the race. Dill says the outside spending and Sen. Olympia Snowe's retirement point toward growing pessimism toward government.