GREER, S.C. – Republican presidential contender Mitt Romney declared Thursday he has paid at least 13 percent of his income in federal taxes every year for the past decade, offering that new detail while still decrying a “small-minded” fascination over returns he will not release. President Barack Obama’s campaign shot back in doubt: “Prove it.”

Campaigning separately, Romney and running mate Paul Ryan also scrambled to explain their views on overhauling Medicare, the health care program relied on by millions of seniors.

Romney, the former company CEO, set up a whiteboard to make his case with a marker, while lawmaker Ryan resorted to congressional process language to explain why his budget plan includes the same $700 billion Medicare cut that he and Romney are assailing Obama for endorsing.