Sen. Barack Obama offers himself as a post-partisan uniter who will solve the country's problems by reaching across the aisle and beyond the framework of liberal and conservative labels he rejects as useless and outdated.

But as Obama heads into the final presidential primaries, Sen. John McCain and other Republicans have already started to brand him a standard-order left-winger, "a down-the-line liberal," as McCain strategist Charles R. Black Jr. put it, in a long line of Democratic White House hopefuls.

Sen. Hillary Rodham Clinton's campaign has also started slapping the L-word on Obama, warning that his appeal among moderate voters will diminish as they become more aware of liberal positions he took in the past, such as calling for single-payer health care and an end to the U.S. embargo against Cuba. "The evidence is that the more [voters] have been learning about him, the more his coalition has been shrinking," Clinton strategist Mark Penn said.

The double-barreled attack has presented Democratic voters with some persistent questions about Obama: Just how liberal is he? And even if he truly is a new kind of candidate, can he avoid being pigeonholed with an old label under sustained assault?