Mr. Romney, who made millions by running Bain Capital, a private equity firm, paid an effective federal tax rate of 14.1 percent in taxes, the campaign said, primarily because most of his income was in the form of capital gains that are taxed at a lower rate than ordinary income. Mr. Romney has said that he has paid at least 13 percent in federal income taxes in each of the last 10 years.

In order for that claim to be true in 2011, Mr. Romney had to voluntarily take a smaller deduction than he was entitled to for his charitable deductions, his advisers said Friday.

The campaign said Mr. Romney and his wife, Ann, donated about $4 million to charity in 2011, but claimed only $2.25 million as a deduction. The campaign said that Mr. Romney’s tax liability would have been far lower in 2011 had the Romney’s claimed all the full deduction for their charitable contributions.

“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 percent in income taxes in each of the last 10 years,” said R. Bradford Malt, Mr. Romney’s trust.

It is possible, however, that Mr. Rommey could still deduct the unclaimed amount of his charitable donations in future tax years, experts said.