The campaign also promised to release a summary showing that the Romneys paid taxes for each year between 1990 and 2009 at an average effective rate of 20.2 percent, and that the lowest tax rate during that period was 13.6 percent.

Romney, former governor of Massachusetts and a multimillionaire former private equity investor, has faced severe criticism throughout the race for refusing to release more tax data.

Democrats have sought to brand him a vastly wealthy plutocrat who is out of touch with everyday Americans and too secretive about his own taxes.

He released his 2010 tax returns, and promised he would do the same with 2011 before his October filing deadline, but he has repeatedly stressed he was not likely to release full tax data for years prior to 2010.

The latest filing, as well as the 20-year summary conducted and notarized by PricewaterhouseCoopers, will be available for public view online at 3:00 pm (1900 GMT) at the website www.mittromney.com/disclosure.

In addition, Brad Malt, the trustee of the Romney's blind trust, said in a blog post that "the Romneys donated $4,020,772 to charity in 2011, amounting to nearly 30 percent of their income."

"The Romneys thus limited their deduction of charitable contributions to conform to the Governor's statement in August, based upon the January estimate of income, that he paid at least 13 percent in income taxes in each of the last 10 years," Malt added.