Sens. Barack Obama, D-Ill., and Hillary Clinton, D-N.Y., were feverishly trying to get their message out to voters in the Keystone State, where 158 delegates are up for grabs, ABC's "Good Morning America" reported Monday.

Obama, in what observers say is his most negative advertising of the primary season, cast his opponent as a compromised Washington insider.

Clinton responded by suggesting Obama's message has become old-fashioned politics, asking Democrats to look harder at the two contenders for the Democratic presidential nomination, The New York Times reported.

Obama, hoping to lock up the nomination, outspent Clinton 2-to-1 on television advertising in Pennsylvania, with commercials challenging her healthcare plan and hinting she was beholden to special interests.