“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.

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.

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.