WASHINGTON (Reuters) - Republican presidential nominee Mitt Romney on Sunday denounced last year's bipartisan debt-ceiling deal that helped avert an unprecedented U.S. government debt default even though one of its backers was his running mate, Paul Ryan.

Without mentioning Ryan by name, Romney called the agreement between the White House and top congressional Republicans "a big mistake," citing potential big defense spending cuts that could come as part of the deal.

Lawmakers reached an 11th-hour deal in August 2011 after a huge showdown, agreeing on nearly $1 trillion in federal spending cuts over 10 years with the promise to impose another $1.2 trillion to reduce budget deficits. The deal was coupled with an increase in the Treasury Department's borrowing authority, as it had bumped up against a legal limit.

As head of the House of Representatives Budget Committee, Ryan backed last year's deal.

"I thought it was a mistake on the part of the White House to propose it (and) I think it was a mistake for Republicans to go along with it," Romney said of the deal.

"I want to maintain defense spending at the current level of the GDP," Romney told NBC's "Meet the Press" in a pre-recorded interview that aired on Sunday. "This sequestration idea of the White House, which is cutting our defense, I think is an extraordinary miscalculation."

Unless an agreement is reached by year's end to implement the same amount in deficit reduction, automatic spending cuts - known as "sequestration" - of $1.2 trillion over 10 years, with a big portion in defense spending, would begin January 2.

So far, Democrats and Republicans have been unable to do so, amid warnings that the possible automatic spending cuts could help push the nation back into a recession.

Ryan, appearing on CBS's "Face the Nation," said he supported the 2011 deal in a bid to find needed "common ground" with Obama and other Democrats. Ryan said Republicans have since proposed ways to reduce "wasteful Washington spending" and avert $1.2 trillion in automatic cuts.

Ryan said that "President Obama has done nothing" to help reach an agreement to reduce spending while also protecting the economy. The sluggish U.S. economy is the central issue ahead of the November 6 elections.

'MORE THAN HAPPY'

Obama, in a pre-recorded interview aired on Sunday on CBS's "Face the Nation," said, "I'm more than happy to work with Republicans" to cut such a deal.

But he repeated his insistence that deficit reduction include revenue increases as well as spending cuts. "We can make sure that we cut two-and-a-half dollars for every dollar of increased revenue," Obama said. Republicans have opposed any increased revenue from tax hikes, helping create a stalemate.

Romney and Ryan blasted Obama for failing to meet a deadline last week, set by a new law, to explain how he would begin implementing the automatic cuts.

"He has violated the law that he, in fact, signed," Romney said. "The American people need to understand how it is that our defense is going to be so badly cut."

Ryan, appearing on ABC's "This Week" program, denied that Romney's proposal to cut taxes and eliminate yet-to-be-identified tax loopholes amounts to a "secret plan."

"No, no, no, no. What we don't want is a secret plan," Ryan said, explaining that the details are not being kept under cover but instead have not yet been worked out. "The best way to do this is to show the framework, show the outlines of these plans, and then to work with Congress to do this," Ryan added.

Ryan suggested that eliminating tax loopholes would hit higher income people harder.

Former President Bill Clinton told last week's Democratic presidential convention that Romney's plan to reduce taxes by $5 trillion would require the government to cut programs that benefit middle-class families and poor children.

"Those claims have been pretty discredited," Ryan said, pointing to analyses he said show the tax plan would work.

"We think the secret to economic growth is lower tax rates for families and successful small businesses by plugging loopholes," Ryan said.

In his "Meet the Press" interview, Romney followed up on Ryan's comments on Friday that another round of monetary stimulus from the U.S. Federal Reserve would be a bad idea. Romney also forecast high inflation "down the road."

"Well, I don't think that easing monetary policy is going to make a significant difference in the job market right now. I think what the nation needs is a change in fiscal policy, a different structure to our economic positions," Romney said.

The comments by Romney and Ryan come ahead of the Fed's September 12-13 meeting at which some economists think the policymakers will unveil another round of bond buying to prop up the country's weak economic recovery.

"And of course the stock market does well in part because the indication by the Fed that they're going to print more money, pour more money into the system, says we're likely to have, down the road, high inflation," Romney said.

"And where else are you going to go? If interest rates are going to be near zero, investors have to go somewhere to protect against inflation. And the stock market's the only place to go," Romney added.

Romney already has said he would not reappoint Fed Chairman Ben Bernanke to a third term. Bernanke's second four-year term as chairman at the Fed ends January 31, 2014.