MILWAUKEE (Reuters) - Barack Obama has surged past Hillary Clinton to open a big national lead in the Democratic presidential race, according to a Reuters/Zogby poll released on Wednesday. Obama also leads Republican front-runner John McCain in a potential November election match-up while Clinton trails McCain, enhancing Obama's argument he is the Democrat with the best shot at capturing the White House. Among Republicans, McCain has a substantial national lead over his last major challenger, former Arkansas Gov. Mike Huckabee, as he takes his final steps toward clinching the nomination.