Calling it a first in the state’s political history, independent U.S. Senate candidate Angus King on Monday released seven years worth of federal tax returns showing that he and his wife earned on average nearly $570,000 a year and paid average federal income taxes of 22.2 percent.

King’s tax returns also showed that he and his wife, Mary Herman, made average charitable contributions of 13.4 percent, or about $76,000 per year, with most going to Maine charities.

“It’s all there. We’re being fully transparent,” King campaign spokeswoman Crystal Canney said.

King’s campaign, which believes the release of his personal financial records is the widest in the history of Maine politics, responded to a challenge by Democrat Cynthia Dill in the race to fill Sen. Olympia Snowe’s soon-to-beempty seat. Dill had urged the three leading candidates to release 10 years’ worth of tax returns through a trusted third party.

King released his documents — hundreds of pages — through his campaign instead of through a third party. Seven years’ worth of tax returns is all that his accountant kept and all that’s required by the Internal Revenue Service, Canney said. King didn’t keep the older returns, she said.

Summers and Dill earned far less than King, who’s a self-made millionaire.

Summers and his wife averaged nearly $91,000 in annual income over eight years of federal tax returns that the campaign provided Monday night. Dill and her husband reported about $65,000 in average annual income over 10 years worth of tax returns provided.

As for King, the majority of his wealth comes from the sale of his energy conservation company, Northeast Energy Management, before running for governor. Much of his earnings over the seven-year period came from stocks and bonds.

Over that period, the family’s gross income ranged from $490,486 to $665,485, according to his accountant. The federal tax rate on his earnings fluctuated between 20 percent and 26.4 percent, which reflected the fact that much of his income came from capital gains on the stock sales, which are taxed at 15 percent. King’s capital gains averaged $231,206 during the period.

At a recent debate in Lewiston, King advocated for raising the federal capital gains tax rate to make it the same as taxes for other income, a position on which Dill agrees with him. Summers, in contrast, proposed eliminating the capital gains tax altogether to encourage people to reinvest the earnings in the economy.

The documents show King and Herman contributed thousands of dollars to charities, often in the form of stock transfers.

Beneficiaries included Guatemalan charity Safe Passage, the Maine Community Foundation, the University of New England, and a school building fund on North Haven Island, where U.S. Rep. Chellie Pingree has a home. Other donations included Family Crisis Services Endowment and Family Planning Association of Maine.

Dill’s proposal came against a backdrop of a similar debate in the presidential race in which Republican Mitt Romney refused to release returns before 2010.

She said Monday that millions of dollars have been spent on deceptive advertising in the Senate race and that releasing tax returns helps to promote accountability.

Three other independents in the race — Steve Woods of Yarmouth, Danny Dalton of Brunswick and Andrew Ian Dodge of Harpswell — were not included in Dill’s proposal.