FactCheck.org : Romney and the Tax Return Precedent
In more than three decades, no other nominees for either party have released fewer than five years’ worth of returns. Romney’s own father released a dozen years’ worth when he ran for the GOP nomination in 1968.
“There is no standard,” said Joseph J. Thorndike, director of the Tax History Project at Tax Analysts, a nonprofit, nonpartisan publisher of tax information. “Some candidates have released a lot more returns and others about the same number. It depends on the particular politics at the time and if they are trying to pressure the people they are running against to release more of their returns. Someone will release a lot of returns when they think they have an advantage over their opponent.”
In 2008, for example, he said, Barack Obama released seven years of tax returns to pressure then-Democratic primary opponent Hillary Clinton to follow suit.