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.

Heading into crucial March 4 nominating contests in Ohio and Texas, Obama has gained the upper hand in a close and fierce Democratic duel with Clinton. McCain broke open the Republican race and has driven out most of his leading rivals.