Sen. Barack Obama set out this morning to puncture the idea of a Clinton-Obama ticket, calling the suggestion an example of "gamesmanship" and "doubletalk" by Sen. Hillary Rodham Clinton and former president Bill Clinton, who said here Saturday that a ticket with Clinton on top and Obama as vice president would be "an almost unstoppable force."The very idea is presumptuous, Obama said, pointing out that he has won more states, more votes and more Democratic delegates than Clinton. He also said, with evident delight, that their notion undermines the Clintons's central challenge to his candidacy -- that he is not prepared to be president. "I don't understand," Obama said, playing the moment as the crowd cheered, "if I'm not ready, how is it you think I would be such a great vice president?"