"I'm not in favor of financing tax cuts with borrowed money," Greenspan said during an interview with Bloomberg Television. "I always have tied tax cuts to spending."
McCain has said that he would offset his proposed cuts - including reducing the corporate tax rate and eliminating the Alternative Minimum Tax that has plagued middle-class families - by ending congressional pork-barrel spending, unnecessary government programs and overhauling entitlement programs such as Medicare and Social Security.
Democrats pounced on Greenspan's comments, in part because McCain professed last year that he was weaker on economics than foreign affairs and was reading Greenspan's memoir, "The Age of Turbulence," to educate himself.
"Obviously he needs to go back to that book and study it some more," Sen. Claire McCaskill, D-Mo., said during a conference call arranged by the campaign of Democratic nominee Barack Obama. McCaskill said eliminating congressional earmark spending - estimated at $17 billion annually - cannot offset McCain's proposed tax cuts.
"That's a huge amount of money, but it's not even a drop in the bucket to pay for $3.5 trillion in tax cuts," she said. "So, every time he throws up earmarks and he's asked how he's going to pay for it, he knows he's being disingenuous, he knows he's not being forthcoming."
McCain campaign officials dispute the $3.3 trillion figure, saying it assumes eliminating 2003 tax cuts made by the Bush administration and then cutting from that higher level. They say McCain is proposing tax cuts worth $600 billion from current levels.
"John McCain opposed President Bush's tax cuts in 2003, because they didn't include the necessary spending controls. Sen. McCain's proposed job-growing tax cuts are modest in comparison to his plans to slow the exploding growth of federal expenditures - meaning that contrary to Chairman Greenspan's assertions, this relief isn't proposed on borrowed money," said McCain spokesman Tucker Bounds.
While McCain opposed the 2003 cuts and previous Bush administration tax cuts from 2001, he now says he would leave them intact. Obama has said he would repeal Bush tax cuts benefiting families making over $250,000 annually to pay for programs and provide middle-tax class relief.
Meanwhile, organizers of a conservative summit in Washington said McCain and his running mate, Alaska Gov. Sarah Palin, missed an opportunity by not addressing the gathering. Some 2,100 activists from 44 states, plus another 10,000 people who signed up to watch online, participated in the three-day Values Voter Summit.
On Saturday, McCain was less than 10 miles away, working in at his campaign headquarters in Arlington, Va. Palin was leaving Alaska and traveling to a rally in Reno, Nev. Last year, McCain and seven other GOP presidential candidates spoke at the summit.
"I think there is some disappointment that he's not here. I think there's greater disappointment that Palin is not here," said Tony Perkins, president of the Family Research Council, a key sponsor of the summit. "I think people would have liked to have heard from her."
Activists attending the summit were unanimous in their enthusiasm for Palin, including several who said their support for McCain was lukewarm before he selected her.
Gary Ward, pastor of the Rocky Point Church in Stephenville, Texas, said he supported former Arkansas Gov. Mike Huckabee for the GOP nomination but that his enthusiasm for McCain has been increased by his choice of Palin and his recent statement that he believes life begins at conception.
"That was absolutely the right answer," Ward said.
Elizabeth Kish, an administrative assistant from Gainesville, Fla., said she was put off by McCain's record on immigration and was considering voting for Libertarian Party candidate Bob Barr until Palin's selection.
"Once he chose Palin that was it for me," said Kish, who was wearing a "Pro-Life Pro-Palin" button and another button featuring pictures of Chief Justice John Roberts and Supreme Court Justice Samuel Alito under the slogan, "The Kind of Change I Believe In."
Associated Press writer Joan Lowy contributed to this report.