Looking to blunt relentless Democratic attacks, Mitt Romney on Friday released his 2011 tax return, a summary of his effective tax rates for the past two decades and, for good measure, two doctor's notes attesting to the good physical health of the candidate and his running mate, Paul Ryan.

The tax return was released at 3 p.m. ET on the website www.mittromney.com/disclosure. Ahead of the release, the former Massachusetts governor's campaign released a blog post summarizing the document.

Releasing information on a Friday afternoon is traditionally a way to reduce the amount of media exposure.

The move fulfills a promise Romney made earlier in the 2012 presidential campaign. But it was unlikely to quiet Democratic criticisms that Romney has failed to live up to a standard set by his father, former Michigan Gov. George Romney, who released 12 years of tax returns when he ran for president in 1968.

The Romneys paid $1,935,708 in taxes on $13,696,951 of mostly investment income for an effective tax rate of 14.1 percent. (The Obamas paid an effective tax rate of 20.5 percent in 2011, a lower rate than the president's secretary, according to the White House.)

Romney, who is thought to have a personal fortune in the neighborhood of $250 million, gave $4,020,772 to charity, 30 percent of their income. (The Obamas gave 21.8 percent of their income to charity.)

The blog post, written by the manager of Romney's blind trust since 2003, R. Bradford Malt, said the Romneys had filed their 2011 tax return with the IRS Friday morning. It also indicated that the Romneys' tax preparer, PricewaterhouseCoopers, would provide a letter summarizing the tax rates that the Romneys paid from 1990 to 2009.

The Romneys paid an average annual effective federal tax rate of 20.2 percent, with the lowest rate coming in at 13.66 percent. Over that same stretch of time, they gave an average of 13.45 percent of their adjusted gross income to charity.

And Romney paid a far lower rate than the top 35 percent tax rate levied on the largest salaries because most of his income came from investments, which are taxed at far lower rates.

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"During the 20-year period covered by the PWC letter, Gov. and Mrs. Romney paid 100 percent of the taxes that they owed," the blog post read.

Democrats led by the Obama campaign have repeatedly hit Romney over his refusal to disclose his tax returns—a fight that has helped to keep Romney's vast wealth in the media spotlight at a time when the president is trying to paint him as an out-of-touch millionaire bent on helping the wealthy.

Romney paid a price—literally—for saying that he paid at least 13 percent in federal income taxes over the past decade. He and his wife did not take the full deductions to which they were entitled for their charitable giving.

"The Romneys' generous charitable donations in 2011 would have significantly reduced their tax obligation for the year," Malt wrote. "The Romneys thus limited their deduction of charitable contributions to conform to the Governor's statement in August, based upon the January estimate of income, that he paid at least 13% in income taxes in each of the last 10 years."

Team Obama showed no sign of letting up in its attacks, with deputy campaign manager Stephanie Cutter saying in a statement that Romney paid a lower tax rate than some middle-class families "because of a set of complex loopholes and tax shelters only available to those at the top."

Cutter charged that the 2011 return "continues to mask Romney's true wealth and income from Bain Capital, leaving the American people in the dark about critical details about his finances."

"Why does Mitt Romney not just release the full returns, instead of the bare summary he has provided of the last 20 years, so voters can make their own judgments about Mitt Romney's finances?" Cutter asked.