AUGUSTA — Former Gov. Angus King officially entered the U.S. Senate race this morning, filing 6,000 signatures with the Maine Secretary of State’s Office.

King has been the presumed frontrunner since shortly after Sen. Olympia Snowe, R-Maine, announced in February she would retire. He is running as an independent and has refused to say which party he would side with if he become a tie-breaking vote in the Senate. Republicans and Democrats have criticized the stand and said King would be ineffective without a party to support him.

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King told a crowd of supporters at the State House that he doesn’t think he can fix the polarized U.S. Senate as a lone independent, but that he could help build a core centrist coalition that breaks through the partisanship. “All it would take is four or five” senators, he said.

King also said he isn’t taking the race for granted. “This is going to be an all-out campaign for the next six months,” he said.

Six Republican candidates and four Democrats are hoping to run against King. The parties will select their nominees at the June 12 statewide primary election. There also are at least two other independents in the race.