In one of the toughest attacks of the 2012 campaign, a pro-Obama super PAC released a battleground ad Tuesday that attempts to link Mitt Romney to the death of a former Kansas City steelworker's wife. 

Romney's campaign responded harshly, calling the ad a "contemptible effort" to conceal President Obama's economic record. 

But the ad marks a steady escalation in recent weeks in the charges being flung by both sides -- particularly by the super PACs, which are not technically an organ of the campaigns. 

The Priorities USA ad focuses on the story of Joe Soptic, a former worker at GST Steel. He was among those affected when the firm declared bankruptcy in 2001, following the takeover by Romney's Bain Capital firm and other companies. 

"I don't think Mitt Romney understands what he's done to people's lives by closing the plant," Soptic said in the ad. He recalled how when "Mitt Romney and Bain" closed the plant, his family lost their health care.