"I think they know their plan is not very popular,” Obama said in a midday speech to a crowd of 3,000 at an amphitheater here. "You can tell that because they are being dishonest about my plan. Especially when it comes to Medicare.”

Medicare emerged swiftly as a major issue in the campaign after Romney’s announced Saturday that Ryan will join him on the ticket. Romney’s campaign has tried to blunt Democratic attacks by accusing Obama of jeopardizing the popular entitlement program.

Obama defended his own Medicare policies Wednesday and charged that Republicans’ criticisms of his actions are a sign of desperation.

“They are throwing everything at the wall to see if it sticks, ” said Obama, who was introduced by his wife, first lady Michelle Obama.

“Here is what you need to know: I have strengthened Medicare. I made reforms that saved millions of seniors hundreds of dollars on their prescription drugs, ” he said. “I proposed reforms by getting rid of wasteful spending in the health care system, reforms that will not touch your Medicare benefits, not by a dime. ”

“Mr. Romney and his running might have a different plan, ” Obama said. “They want to turn Medicare into a voucher program. That means seniors would not have the guarantee of Medicare. ” Obama said Romney’s plan for the program could force seniors to pay as much as $۶,۴۰۰ more a year for their health care.

Earlier Wednesday, Romney distanced himself from the $۷۰۰ billion in Medicare spending cuts Ryan proposed in his budget, while attacking Obama for including projected cuts to the program of about the same amount. “My campaign has made it very clear: The president’s cuts of $۷۱۶ billion to Medicare — those cuts are to be restored if I become president and Paul Ryan becomes vice president, ” Romney said on “CBS This Morning. ”

Obama did transfer money from Medicare to other purposes in his health care law, but in ways that his backers argue ultimately reinvest in the program. Some of the money goes to health care for non - elderly people, but some of it goes to closing the prescription drug “donut hole” and funding preventive care in Medicare, among other measures.

Romney spokesman Ryan Williams responded to the president's speech by saying that Obama "has a long history of launching shameful political attacks on Medicare - but he's the only person in the race who has actually cut Medicare. & As president, Mitt Romney will always protect this vital program for seniors and strengthen it for future generations.”