Charles Dharapak, Pool, Associated Press
Texas Gov. Rick Perry, left, speaks as former Pennsylvania Sen. Rick Santorum, and former Massachusetts Gov. Mitt Romney listen at the South Carolina Republican presidential candidate debate in Myrtle Beach, S.C., Monday, Jan. 16, 2012.

WASHINGTON — Mitt Romney's promise to release his 2011 tax return in April follows the practice of leading presidential candidates that began after Watergate. If history is any lesson, questions and criticism will continue long afterward.

For more than three decades, the major party nominees have released their income tax records. Some offered one year and others more than 10 years of returns. The same has been true for vice presidential candidates, except for Sen. Bob Dole of Kansas when he joined President Gerald Ford on the GOP ticket in 1976.

Former California Gov. Ronald Reagan broke his longstanding rule of keeping his personal finances private in July 1980 when he released his 23-page 1979 income tax return weeks before accepting the GOP nomination. Sen. Hillary Rodham Clinton weathered weeks of criticism in 2008 for not releasing tax returns showing family income after Bill Clinton left the presidency. She ultimately produced the records in April of that year after taking a pounding from her top party rival, Sen. Barack Obama.

Even after returns are released, controversies persist. Democratic Sen. John Kerry took heat in 2004 after releasing his returns because his wife, heiress to the Heinz Co. food fortune, initially refused to release hers, which were filed separately. Four years later, Republican Sen. John McCain faced similar criticism because his wife didn't release her separately filed returns, which reflected income from a Phoenix-based beer distributing company she inherited. She later released the two top summary pages of one year's return, the same that Kerry's wife had released.

Now it's Romney's turn. Texas Gov. Rick Perry, who released his own tax returns dating back to 1991, urged Romney during a debate Monday to release his. Romney, whose net worth is estimated at roughly $250 million, previously had resisted calls to release his tax returns. Anticipating a key question about his taxes, Romney disclosed Tuesday that he pays an effective federal tax rate of about 15 percent, still higher than the rate paid by many Americans.

Romney said in the debate that he will decide whether to release returns in the coming months.

"I hadn't planned on releasing tax records because the law requires us to release all of our assets, all the things we own. That I have already released," he said. Later, he added, "What's happened in history is people have released them in about April of the coming year and that's probably what I would do."

Romney's right. There is no law requiring presidential candidates to release personal tax information. Since 1978, however, they've had to disclose information about their income and some about their assets, like real estate holdings, investments and outside business interests. But those disclosures only show a range of values for assets, making it impossible to use those forms to identify a candidate's actual wealth.

He's also right that many leading candidates in recent history chose April to release their tax returns.

On Tuesday, Romney gave reporters in South Carolina more insight into his plans, saying he would release one year of his tax returns, not the six previous years that Obama released as a candidate in 2008 or even the two years that McCain released that year.

"People will want to see the most recent year," Romney said.

He said he's paid "closer to the 15 percent rate" in taxes because most of his income has come from investments and not ordinary wages, which have a top tax rate of 35 percent for those with the highest earned income.

When he ran in 2008 Romney refused to release his tax returns, and he previously had filed only state financial disclosures that described his assets in the most general terms.

Former House Speaker Newt Gingrich of Georgia has said he will release his tax returns Thursday.

Even when tax returns are released, they offer only a narrow snapshot into a candidate's financial background. But some candidates in the past, like former Georgia Gov. Jimmy Carter in 1976, have offered more specific breakdowns of their financial worth.

Some of the bigger controversies over tax returns have belonged to vice presidential candidates. Geraldine Ferraro was a little known New York City congresswoman when the Democratic nominee, Sen. Walter Mondale, chose her as his running mate in 1984. The euphoria of that history-making selection of a woman for a major presidential ticket ended abruptly when Ferraro began battling criticism over her husband's refusal to release his separately filed tax returns. After they finally relented, she faced more controversy over accounting errors and other questions in the returns.

George H.W. Bush was Reagan's vice president at the time and became one of Ferraro's biggest critics on the issue, only to face his own controversy that year when he initially declined to release three years of returns. Bush argued he couldn't release them because he had turned his personal financial affairs over to a blind trust when he became vice president. But he ultimately released the returns weeks before the 1984 election.

Dick Cheney, George W. Bush's running mate in 2000, got blasted after releasing his returns for his financial ties to a Dallas-based oil company and for charitable giving that amounted to less than 1 percent of his income. After becoming Obama's running mate in 2008, Sen. Joe Biden faced similar criticism over his charitable giving once his tax returns became public.

In 2008, Alaska Gov. Sarah Palin's tax returns for two years triggered scrutiny because Palin, then McCain's running mate, did not list per diem payments the state made to her when she stayed in her own home. She later had to pay taxes on the payments.