In an interview on Saturday, Republican vice presidential candidate Paul Ryan said his Medicare proposal had "evolved" - with, among other influences, input from Democrats.

"As we got into looking at this issue, talking with Democrats, it evolved to be a plan that is now bipartisan," the House Budget Committee chairman said in an interview with Fox News, referring to a compromise proposal he and Sen. Ron Wyden, a Democrat from Oregon, unveiled last December. Neither of Ryan's budgets, with proposals for Medicare, have passed the Democrat-controlled Senate.

But now, as both Ryan and GOP presumptive presidential nominee Mitt Romney criticize President Barack Obama for his health care reform law, Democrats are using at least one part of the proposal to knock the congressman from Wisconsin in return.

"I heard Mitt Romney deride the $700 billion cuts in Medicare that the president achieved through health care reform," Obama's deputy campaign manager, Stephanie Cutter, said last Sunday on CBS' Face the Nation. "And interestingly enough, Paul Ryan protected those cuts in his budget."

At his rally in The Villages, Florida, earlier, Ryan leveled the charge to which Cutter was referring.

"Here's what the president won't tell you about his Medicare plan, about Obamacare," Ryan said. "The president raids $716 billion from the Medicare program to pay for the Obamacare program."

The non-partisan fact-checker Politifact.com has rated a nearly identical charge by Romney, that Obama has "robbed" Medicare of the amount as "mostly false." It also found Cutter's assessment of the similarities - but not the description of reductions as "cuts" - between the Romney and Obama's plans to be "true."

Asked in the Saturday interview to respond to that general point, Ryan said, "Well there's a lot to sort through. Look, we're offering leadership, we're offering solutions. The president is not. The president has a failed economic agenda - 23 million people are still looking for work."

In a July interview with ABC News, Ryan explained that the difference between his plan and Obama's plan with regards to the $716 billion was how the money would be used.

"Well, our budget keeps that money for Medicare to extend its solvency," he said. "What Obamacare does is it takes that money from Medicare to spend on Obamacare."

Since Ryan was introduced to the Republican ticket a week ago, Democrats have criticized his budget and Medicare proposal. In an interview with a CNN affiliate in Wisconsin, WBAY, this week, Romney said, "Paul Ryan and my plan for Medicare, I think, is the same if not identical - it's probably close to identical."

In a campaign memo circulated on Saturday, Romney's policy director, Lanhee Chen, explained that both candidates support a plan which "(a) makes no changes to Medicare for those over 55; (b) starting in 2022, transitions Medicare to a premium support model with competitive bidding to determine support levels, (c) offers traditional Medicare as an option competing in that system, and (d) means-tests the premium support so that lower-income seniors receive the most generous assistance."

In his Fox interview, Ryan also echoed what he said in a Thursday paper statement, when he said he took responsibility for letters he signed and sent to federal agencies in support of stimulus-funded grants requested by a non-profit organization in his state.

"My office sends tens of thousands of letters to various federal agencies and this goes through what we call a case work system and we treat it as a case work for constituents," he said. "I take full responsibility for it. It wasn't my intention to send letters supporting stimulus. But this doesn't change the fact that the stimulus was a bomb, it didn't work, it's a boondoggle. Borrowing and spending, spending money on things like Solyndra, that doesn't create jobs."

Ryan had previously said he did not request stimulus funds, and said on Thursday that he was previously unaware of the letters.