White House hopefuls Hillary Clinton and Barack Obama have traded jibes in a surge of negative campaigning ahead of a key Democratic vote in Pennsylvania.During weekend rallies, she accused him of cheering on Republican rival John McCain, while he attacked her for "slash and burn" politics.
Tuesday's primary may prove vital in the drawn-out fight for the nomination.
Mrs Clinton needs a strong victory to stay in the race, but polls suggest Mr Obama has cut into her lead.
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