President Obama offered a sharp critique of his Republican rivals’ plans to revamp Medicare on Friday, telling a conference of senior citizens that their move to privatize the federal entitlement program will increase costs.

"I don’t consider this approach bold or particularly courageous,” Obama said. “I just think it’s a bad idea. No American should spend their golden years at the mercy of insurance companies.”

Obama spoke to the AARP’s annual conference by video feed from Woodbridge, Va., where he was later to appear at a campaign rally. The president’s remarks came shortly before Rep. Paul Ryan (R-Wisc.), who is running for vice president on the ticket of GOP presidential nominee Mitt Romney, was to address the same audience.

Romney has adopted Ryan’s plans to turn Medicare into a voucher system, which they have said will reduce costs by allowing private companies to compete to offer services. However, independent analysis have disputed their contention, saying costs could rise over the next couple decades if the system is implemented.

Seniors are a key constituency for both parties and could be critical in Florida, the largest swing state with 29 electoral votes. 

Obama has said he will keep Medicare, and he offered a defense of his approach during the video address.

 “We do have to reform and strengthen Medicare for the long haul, but we're going to do it by reducing the costs of care,” the president said.

 Obama took pains to pre-empt an attack from Ryan that the administration cut $716 billion from Medicare to help pay for the president’s health care reform bill. Ryan has made that claim on the trail to suggest that the White House has no ground to stand on to criticize the Republican plan. But Obama has argued that the reduction was achieved by eliminating waste and fraud by insurance companies.

“When you hear the notion that we somehow took $716 billion and robbed it from Medicare beneficiaries, I want you to know that simply not true,” he said.

Asked about the intractable gridlock in Washington, Obama said he was willing to work with Republicans. But he added that: "Part of what you want from a president, is someone who is working hard to bring people together, but who is also willing to stand up to bad ideas that end up tilting the playing field further toward those who have already made it instead of people trying to make it."