The Bernie Sanders campaign is trying to take advantage of her relative weakness on the issue, releasing an advertisement today spotlighting his blanket opposition to fracking.

Since early April, the two candidates have engaged in a back-and-forth over Clinton’s contributions from the fossil fuel industry. She says she has received little, while Sanders and Greenpeace say she’s raised over $4 million. Clinton has responded that she’s sick of Sanders lying about her record.

The advertisement implies these contributions are the reason her stance on fracking is less than clear. In a March debate, Clinton said she would place so many conditions on fracking that it would be allowed practically nowhere in the United States. Clinton has also said she supports New York’s fracking ban. But while campaigning in New York for Sanders, Gasland director Josh Fox reminded voters of Clinton’s support for expanding natural gas abroad while she was secretary of state.

As the April 19 primary approaches, fracking and fossil fuel usage could be important issues for Sanders—the majority of the state agreed with Governor Andrew Cuomo’s decision to ban fracking in the state.