President Barack Obama used his regular weekly radio address Saturday to continue pounding away at Republican plans to overhaul Medicare. The address underscored the new prominence Medicare has assumed as a campaign issue in the past two weeks. Paul Ryan, Republican, Mitt Romney's runnign mate, has championed an alternative to the government-run Medicare program that would provide vouchers to older Americans for the purchase of private health insurance. "I'm willing to work with anyone to keep improving the current system, but I refuse to do anything that undermines the basic idea of Medicare as a guarantee for seniors who get sick," Obama said.