As Democrats move to tie Mitt Romney to his newly minted running mate's proposal to revamp Medicare as a voucher system for Americans currently under 55, the presumptive Republican presidential nominee has released a new TV ad that goes on offense against Obama on Medicare.

“Obama has cut $716 billion dollars from Medicare," says the narrator of the Romney campaign ad. "Why? To pay for Obamacare.”

The assertion is based on a Congressional Budget Office estimate from July that spending for Medicare "would increase by an estimated $716 billion" over the next nine years under an effort to repeal the federal health-care law.

The Obama campaign responded by calling the Romney attack ad hypocritical.

“Mitt Romney’s ad is dishonest and hypocritical. The savings his ad attacks do not cut a single guaranteed Medicare benefit, and Mitt Romney embraced the very same savings when he promised he’d sign Paul Ryan’s budget," said Obama spokeswoman Lis Smith in response to the ad. "Because the President is eliminating subsidies to insurance companies and cutting waste and fraud, we’ve extended the life of Medicare by eight years."

The Romney campaign's engagement with the Medicare issue and the Obama campaign's effort to keep the debate active illustrates how the addition of Rep. Paul Ryan (R-Wis.) — who is often associated with his own controversial Medicare plan — to the ticket has swiftly elevated the debate over Medicare to the forefront of the campaign.