WASHINGTON (AP) — A group backed by conservative billionaires Charles and David Koch is rushing onto the air in Louisiana to help Republican Bill Cassidy's runoff campaign against Democratic Sen. Mary Landrieu.

Freedom Partners Action Fund said late Tuesday that it was starting a television ad blitz against Landrieu, arguing that she is working against Louisiana oil and gas producers. The group plans $2.1 million in ads against Landrieu starting Wednesday.

The group, which formed in June, is on track to spend $25 million this year.

Neither Landrieu nor Cassidy won 50 percent of the vote in Tuesday's three-way race that also featured independent Rob Maness. The race becomes a two-way contest between Landrieu and Cassidy.

Airwaves ahead of the Dec. 6 runoff are expected to be crowded with outside groups' ads.