The possibility that a potential Democratic presidential primary matchup between Sens. Hillary Rodham Clinton and Barack Obama could lead to a Clinton-Obama ticket is raising concerns in GOP circles that it might be unbeatable. While Democratic strategists are more skeptical of the success of a ticket composed of two minorities, some Republican advisers to the White House and leading 2008 hopefuls Sen. John McCain and Rudy Giuliani see the ticket as an easy winner built on the enthusiasm it would generate in Democratic circles.Their theory is that Clinton would stand a good chance to pick up the states that Sen....