WASHINGTON (AP)- Sen. Joseph Biden says any Democrat who wants to run for president in 2008 should keep in mind these three words: Hillary Rodham Clinton."I think she'd be incredibly difficult to beat," the Delaware Democrat said Sunday on NBC's "Meet the Press." "I think she is the most difficult obstacle for anyone being the nominee."The former first lady and Democratic senator from New York insists that she plans to run for re-election in 2006. But speculation persists that she might run for president in 2008."She is likely to be the nominee," Biden said. "She'd be the toughest person and I think Hillary Clinton is able to be elected president of the United States."Biden said he is thinking about running again, 20 years after his first failed bid for the White House because "there's a lot at stake."