WASHINGTON - Republican Sen. Joni Ernst's Democratic challenger is hammering her in tweets and a new radio spot for her votes to confirm EPA Administrator Andrew Wheeler, who she says has undermined the federal biofuels program by issuing dozens of exemptions to the Renewable Fuel Standard, Pro's Eric Wolff reports.

Businesswoman Theresa Greenfield, the Democrat trying to unseat Ernst, launched the radio ad Wednesday as part of a statewide buy slamming Ernst for backing Wheeler, who she referred to as a "fossil fuel lobbyist" because of his past work. The attacks followed tweets in which she called for Wheeler to resign and asked Ernst to join her demand.

While Ernst did vote to confirm Wheeler twice, she has a long record of vocally advocating for the RFS, including holding up the confirmation of former EPA assistant administrator Bill Wehrum over what she viewed as the agency's weak support for biofuels.

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