McCain's Senate salary and book royalties amounted to $215,304 in 2006 and $258,800 in 2007. In 2006, the senator paid $72,771 in federal taxes and $84,460 in 2007.

"Since the beginning of their marriage, Sen. McCain and Mrs. McCain have always maintained separate finances," the campaign said in release. "As required by federal law and Senate rules, Mrs. McCain has released significant and extensive financial information through Senate and presidential disclosure forms. In the interest of protecting the privacy of her children, Mrs. McCain will not be releasing her personal tax returns."

Cindy McCain is the heiress to Hensley and Company, a lucrative beer distributor company in Arizona. Her wealth has been estimated in the hundreds of millions of dollars. 

"Having served the greater Phoenix area since 1955, Hensley & Company is widely respected as an exemplary corporate citizen, and makes significant charitable contributions of its own," the release said.

McCain's Democratic rivals, Sens. Barack Obama (Ill.) and Hillary Rodham Clinton (N.Y.), released their tax returns earlier this month.

The Democratic National Committee (DNC) hit McCain in a Thursday afternoon memo, criticizing him for failing to disclose his tax returns since coming to Congress, adding that by only releasing two years of information, McCain is not disclosing enough.

In the memo, the DNC asked if McCain would make his wife's returns available, noting: "Don't forget that in 2004, the RNC [Republican National Committee] repeatedly called on the Kerry campaign to release Mrs. Heinz-Kerry's separately filed returns.

"McCain's aversion to disclosure stands in contrast with the Democratic candidates, both of whom have released full tax returns dating back at least eight years, as has become expected and traditional," the DNC memo said.