Last week, he told a woman in Ohio that he and Barack Obama don't support clean coal and flatly said, "No coal plants here in America."
But campaigning in Pennsylvania coal country on Thursday, the Democratic vice presidential nominee said the government should steer more money to clean coal. The term is used to describe a variety of emerging technologies that burn coal for electricity without producing as much pollution.
It's an issue that resonates with some working-class voters in Pennsylvania, a group that Democratic presidential nominee Barack Obama has had trouble connecting with. Pennsylvania Democrats gave Sen. Hillary Clinton a 10-point victory in the April primary.
Pennsylvania is the nation's No. 4 coal-producing state; the industry employs more than 7,000 people at nearly 800 mines, and many voters come from coal-mining families. The administration of Gov. Ed Rendell has invested millions of dollars in clean coal technology in the state.
The Obama campaign has said it supports clean coal technology, specifically the development of coal-fired plants that capture carbon dioxide emissions and store the gas underground. Obama even mentioned clean coal in his acceptance speech at the Democratic National Convention.