But forgoing the full deductions available to him put him at odds with his own past assertions that he had never paid more taxes than he owed and his statement that if he had done so, “I don’t think I’d be qualified to become president,” as he put it to ABC News in July.

Mr. Romney had pledged to disclose his 2011 return before Election Day, and his campaign said it was filed Friday with the Internal Revenue Service. His aides appear to have judged that any political harm from releasing the new documents — made public on Friday afternoon — would best be timed for the end of a week that had been among the most difficult of his campaign.

While the release of some figures for the previous two decades went beyond what Mr. Romney had signaled he would be willing to disclose, it remained impossible to get a complete picture of his tax liabilities from those years without his returns. Democrats quickly pounced on Mr. Romney’s decision to release only average figures for his 1990-2009 returns, leaving many details of his finances and tax planning unclear.

In a statement, Stephanie Cutter, the deputy campaign manager for President Obama, said that Mr. Romney “continues to fail” the test of full disclosure by releasing only a summary of his earlier returns. Harry Reid, the Senate majority leader, who had accused Mr. Romney of having paid no taxes for a decade, did not repeat his claim on Friday — but did not back down either.

“When will the American people see the returns he filed before he was running for president?” Mr. Reid said in a statement. “Governor Romney is showing us what he does when the public is looking. The true test of his character would be to show what he did when everyone was not looking at his taxes.”

The Romney campaign took questions about the new documents only over e-mail, and a memo from his lawyer, R. Bradford Malt, left unanswered questions that have swirled about Mr. Romney’s overseas income, foreign tax credits and use of sophisticated corporate structures abroad to minimize his tax burdens at home.

A campaign spokeswoman did not respond to questions about which years Mr. Romney or the family trusts had filed separate forms with the Internal Revenue Service disclosing their foreign income. Disclosing those forms would reveal whether Mr. Romney had over the years declared all of his foreign income to the I.R.S. in a timely manner.

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The summary of his returns for the years before 2010 said that the Romneys had owed both federal and state taxes in each year between 1990 and 2009 and had paid an average effective federal income tax rate of 20 percent of their adjusted gross income.

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But accounting experts cautioned that without seeing the returns themselves it was impossible to gauge Mr. Romney’s actual tax burden. The campaign declined to disclose the minimum dollar amount of Mr. Romney’s federal income tax obligations during those two decades.

Citizens for Tax Justice, a liberal-leaning research group, said Friday that by including in the average the years 1992 through 1997, Mr. Romney’s accountants skewed his average rate upward because investment income — the overwhelming source of Mr. Romney’s wealth — was taxed at nearly double the current rate of 15 percent. In addition, the family appeared to defer some tax deductions into future years, a move that would give Mr. Romney further options — all of them legal — to adjust his effective federal tax rate.

In an amended return also released Friday, Representative Paul D. Ryan, Mr. Romney’s running mate, disclosed that he and his wife had initially failed to report $61,122 in income from 2011. He said the failure was inadvertent. The change raised their total income to $323,416 and increased their taxes by $19,917 to $64,674, or 20 percent of adjusted gross income.

They owed a penalty of $59 for the original underpayment. The Ryans explained that they had overlooked their income from the Prudence Little Living Trust. Mrs. Little, who died in 2010, was Mrs. Ryan’s mother.

Some elements of Mr. Romney’s finances became more opaque in 2011. Taxable wages for household employees, which reached $20,603 for four people in 2010, were not included on the 2011 return. Instead, the family made those payments through a payroll company that filed its own return.

Mr. Malt, who manages the family’s trusts, also disposed of politically sensitive investments while Mr. Romney campaigned for president. The 2011 tax returns his campaign released Friday showed that Mr. Romney’s family trusts had invested in shares of a Chinese-owned state oil company and sold those investments last summer, as Mr. Romney’s anti-Chinese comments heated up on the campaign trail.

Mr. Romney’s trusts also hedged against the dollar. Mr. Malt invested in a derivative that would profit if the dollar fell against a group of foreign currencies. He also put some of the family’s money in derivative securities linked to the Japanese stock market and to an index that includes stocks in every major country except the United States.

In 2009 and 2010, the W. Mitt Romney blind trust invested $77,262 in shares of Cnooc Limited, the Chinese state-owned oil company, and the Industrial and Commercial Bank of China. On Aug. 10, 2011, as Mr. Romney was emerging as a harsh critic of China, the shares were sold, producing a profit of $8,138 as the trust made money on the oil company and lost money on the bank.

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Mr. Romney’s campaign has repeatedly criticized Mr. Obama for failing to take a tough line against Chinese trade practices. After Mr. Obama this week announced new trade actions against China, Mr. Romney took credit for forcing his hand.

The Romney family trusts invested around the world. They owned shares in Credit Suisse, the Swiss bank; FLSmidth, a Danish machinery company; ArcelorMittal, a steel company based in Luxembourg with operations around the world; and Komatsu, a Japanese machinery company. All those investments were sold on Aug. 10, 2011 — the day before a Republican primary debate in Iowa.

Mr. Romney’s income in 2011 would put him among those Americans who will most likely pay far higher Medicare taxes next year, thanks to Mr. Obama’s health care law, which Mr. Romney has vowed to repeal.