AUGUSTA, Maine — Independent Senate candidate Angus King’s campaign Tuesday defended the way King’s accountant calculated the former governor’s effective income tax rate on seven years of federal tax returns as Republicans argued that the calculation made it seem like King paid a higher tax rate than he actually did.

Meanwhile, Republican Charlie Summers and Democrat Cynthia Dill released several years of federal tax returns to the public, about a week after Dill called on her two rivals in the race for Maine’s open U.S. Senate seat to release 10 years of federal tax returns.

Summers’ campaign posted eight years of federal and state returns to its website Monday night. Dill’s campaign, meanwhile, encountered technical difficulties in redacting Social Security numbers from scanned copies of the state senator’s tax returns. Ultimately, the campaign released electronic copies of 10 years of tax returns to the Portland Press Herald, from which the newspaper redacted the Social Security numbers of Dill and her family.

The eight years of Summers’ tax returns show Summers and his wife, Ruth Summers, earned about $90,000 on average annually between 2004 and 2011. Summers’ effective tax rate — total taxes paid as a percentage of adjusted gross income — was 8.19 percent. Forbes magazine reported last year that the effective tax rate for people earning between $50,000 and $100,000 in 2008 was 8.41 percent.

In 2011, Summers and his wife earned about $103,000 and paid $8,055 in taxes.

Dill’s 2011 tax return showed she and her husband, Thomas Clarke, earned about $82,000 and paid $1,500, or 1.82 percent of their adjusted gross income, in taxes.

The adjusted gross income reflects income earned after taking certain deductions allowed for members of the military reserves, some business costs, moving expenses and costs teachers incur buying classroom supplies.

Political debate Tuesday, however, centered on how King’s accountant calculated the former governor’s effective income tax rate in documents accompanying the seven years of federal tax returns King’s campaign released Monday.

The accountant, Scott Small of Macdonald Page & Co. in South Portland, calculated the effective rate by dividing total taxes paid by taxable income — the sum remaining after accounting for deductions — rather than calculating total taxes paid as a percentage of adjusted gross income.

As a result, King ended up with an effective rate nearly 7 points higher than he would have under the latter method, which Republican presidential contender Mitt Romney’s accountant used last month when Romney released his 2011 federal tax return.

Small calculated an effective rate of 22.2 percent for King and his wife, Mary Herman, between 2005 and 2011. King’s effective rate would have dropped to 15.41 percent, however, if Small had calculated total tax paid as a percentage of adjusted gross income.

Using the second calculation, Forbes magazine determined that people earning between $500,000 and $1 million — King’s income level — paid, on average, an effective tax rate of 23.92 percent in 2008.

The lowest rate King paid during the span covered by his returns was 12.36 percent in 2010 while the highest was 18.93 percent in 2009. The calculations released by his campaign showed King’s lowest tax rate was 20 percent in 2007; his highest was 26.4 percent in 2009.

Romney’s effective tax rate for 2011 was 14.1 percent, though the Washington Post reported it could have been as low as 10.55 percent if he had deducted all his charitable contributions. While Romney has only released returns for 2010 and 2011, a letter from his accountant says Romney paid an effective tax rate of 20.2 percent between 1990 and 2009.

Maine Republican Party spokesman David Sorensen called the difference in tax rate calculations “the latest incident in an evolving pattern of Angus King trying to mislead Mainers.”

Small, King’s accountant, said there’s no universal definition of effective income tax rate because it’s not something the Internal Revenue Service requires taxpayers to calculate. His accounting software, he said, calculated effective tax rate as total taxes paid as a percentage of taxable income. He also pointed out that the consumer tax guide J.K. Lasser instructs readers to calculate their effective tax rate by dividing total taxes paid into their taxable — not adjusted gross — income.

“There is no rate anywhere in the IRS code that gets applied to adjusted gross income,” he said.

King spokeswoman Crystal Canney said the campaign stood by the tax rate calculations and pointed out that Romney released two years of tax returns, compared with seven released by King.

“They’re not comparing apples to apples here,” she said. “This is just political gamesmanship.”

While not all accountants agree on the definition of effective tax rate, the more common definition used in media reports about Romney, billionaire investor Warren Buffett other high-profile figures generally has been total taxes paid as a percentage of adjusted gross income.

“If you’re using the taxable income, you’re not reflecting the fact that their taxable income might be lower since they’re taking certain deductions,” said Eric Toder, co-director of the Tax Policy Institute in Washington, D.C., a joint initiative of the Brookings Institution and the Urban Institute. “The adjusted gross income isn’t a perfect or complete measure either, but it’s the most complete you have on the return.”