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

The independent candidate has been the presumed frontrunner since shortly after U.S. Sen. Olympia Snowe, R-Maine, announced in February she would retire. King told a crowd of supporters Tuesday morning at the State House he wants to help break down partisan gridlock so the Senate can fix the budget deficit, the health care system and other issues.

“We can’t get to the health care issue if the Senate doesn’t work,” he said.

However, with two weeks to go before the June 12 primary election, it’s clear Republicans and Democrats are not going to roll over for King. The six Republicans running for Snowe’s seat have been especially critical of King during their primary campaigns, calling him a big-spending liberal likely to side with Democrats if elected to Congress. Maine Democrats gather for their state convention this weekend in Augusta and are sure to fire shots at King, too.

Meanwhile, both national parties are expected to invest in the Maine campaign because the winner here could decide the balance of power in the U.S. Senate.

“He cannot cruise in, not with the kind of money that’s going to be brought in here, particularly by the GOP. It’s going to be a tough race,” said Kenneth Palmer, political science professor emeritus at the University of Maine.

“With a seat of national importance, with the numbers in the Senate so close, and with a seat that looked like a (Republican) lock a few months ago, I’m expecting to see a lot of involvement and money from the national parties,” said Ronald Schmidt, associate professor of political science at the University of Southern Maine. “Under those circumstances, we’re probably in for a much more unpredictable race than we’re used to in Maine.”

The National Republican Senatorial Committee issued a statement Tuesday citing King’s donations to President Barack Obama’s re-election campaign.

“The bottom line is the Senate race in Maine will be between two Democrats and a fiscally responsible Republican,” Lance Trover, the group’s spokesman, said in a written statement.

Two Republican candidates also issued statements criticizing King’s refusal to say which party he would caucus with if he is elected to an evenly-divided Senate. King recently told a national television host he might not choose either party, although he won’t go to Washington “to be a potted plant” with no influence on legislation.

Attorney General Bill Schneider criticized King for supporting Obama’s health care reform law and potentially passing up an influential committee assignment rather than caucus with one of the parties.

“It is clear that Governor King is not offering the type of leadership that Maine needs in Washington,” Schneider said in the statement.

Secretary of State Charlie Summers said King already has made it obvious he plans to side with Democrats. “It’s time to come clean,” he said.

King did not declare support for either party Tuesday, but he answered some of the criticism of his record from Republicans and Democrats. He’s not arrogant enough or nave enough to think he can go to Washington by himself and fix things, King told a crowd of supporters at the State House. He said his election would send a strong message to the parties and that he would help build a powerful centrist coalition.

“All it would take is four or five” senators, he said.

King also briefly defended his record as governor, saying he didn’t have a 100 percent success rate but proved wrong the people who said an independent governor could not be effective.

“We were civil,” he said. “We listened.”

Finally, King 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.