Mitt Romney released two years of his own tax returns to the public but that didn't appear to be enough when he vetted running-mate Paul Ryan and other vice presidential candidates.
The campaign team for Romney, the Republican presidential candidate, reviewed several years of tax returns from Ryan and others, according to the head of Romney's VP search process Beth Myers.
But Romney - a former private equity executive with an estimated net worth of up to $250 million - has refused to publicly release more than two years of tax returns.
The secrecy has led Democrats to say he has something to hide about his vast wealth, which included bank accounts in Switzerland, and in the Cayman Islands and Bermuda, known offshore tax havens.
"Why does an American businessman need a Swiss bank account, or investments in known tax havens, if not to be hiding something?" Democratic National Committee Chairwoman Debbie Wasserman Schultz said on "Fox News Sunday."
"Mitt Romney needs to show American voters at least the same number of tax returns that he asked Paul Ryan to show him when he was vetting him for vice president."
Polls show U.S. President Barack Obama has benefited from putting Romney on the defensive over his taxes and wealth, painting him as someone out of touch with middle-class America.
Romney campaign officials declined to comment about the vetting process and how many years of tax returns were required, saying it was meant to be "private and confidential."
It was unclear if Ryan's tax returns, which were stored in a safe in a locked room at Romney's campaign headquarters, will be released to the public.
Tim Pawlenty, the former Minnesota governor who was a potential Romney running mate, said he also had to submit several years of tax returns during the search for a Republican vice presidential candidate.
"So more than two?" asked George Stephanopoulos, the host of ABC's "This Week."
"Well, we don't get into the details of the vetting process, but I gave him a bunch of tax returns," Pawlenty replied. "I don't remember the exact number of years."