Cassidy gave $500 to Landrieu's campaign in 2002, according to Federal Election Commission reports. The donation was first reported by the New Orleans website NoLa Defender.

The donation is part of a pattern of bipartisan giving from the GOP lawmaker. Donations to Democrats include to former Louisiana Gov. Kathleen Blanco (D) in 2003 and 2004.

Cassidy is facing Tea Party candidate Rob Maness (R) as well as Landrieu in the all-party primary, and the unusual donation could undermine his credibility with the base and in attacking Landrieu.

Cassidy argued the donations came before Landrieu moved to the left.

"Mary claimed she was going to represent Louisiana and not D.C. Then she got elected and fell into partisan politics," he said in a statement to The Hill. "Now she votes with Barack ObamaBarack Hussein ObamaOvernight Energy: Dems ask Pruitt to justify first-class travel | Obama EPA chief says reg rollback won't stand | Ex-adviser expects Trump to eventually rejoin Paris accord Overnight Regulation: Trump to take steps to ban bump stocks | Trump eases rules on insurance sold outside of ObamaCare | FCC to officially rescind net neutrality Thursday | Obama EPA chief: Reg rollback won't stand Ex-US ambassador: Mueller is the one who is tough on Russia MORE 97 percent of the time and cast the deciding vote for Obamacare. Now she supports anti-energy candidates who are opposed to our energy industry. Louisiana hasn't left Mary, Mary has left us."

This post was updated at 9:10 p.m.