With the hours left until Pennsylvania voters head to the polls dwindling, Sens. Barack Obama and Hillary Clinton traded attacks over lobbyist ties and negative campaign tactics.

Obama (D-IL) accused Clinton of distorting his record of refusing lobbyist contributions, while painting her as in-the-pocket of Washington special interests, in his most severe television advertisement of the campaign. The ad was responding to a spot from Clinton (D-NY) implying that Obama was a hypocrite for having ties to lobbyists despite a pledge not to take money from them. Clinton has gone hard after Obama, especially since Wednesday night's debate.

"There’ve been times where, you know, if you get elbowed enough, eventually you start elbowing back," Obama told voters in Reading, PA, on Sunday, according to the New York Times.

Clinton responded to a second Obama TV ad about her healthcare plan, with a spot accusing him of going on the attack after stumbling during the debates. Clinton also has launched automated phone calls to voters attacking Obama's healthcare and energy proposals.

The back-and-forth barbs likely will continue through Tuesday's vote, where Clinton needs a strong showing -- beating Obama by at least seven to 12 points, observers say -- to continue her presidential campaign. Clinton has vowed to fight on past Pennsylvania no matter the result, but an unexpected Obama win likely would force her from the race.

This video is from BarackObama.com, broadcast April 20, 2008.

Download videoThis video is from HillaryClinton.com, broadcast April 20, 2008. Download video