A new ad from the Democratic Senate Majority PAC hits Rep. Bill CassidyWilliam (Bill) Morgan CassidyTrump allies see 's---hole' controversy as overblown GOP senator: Leaking Trump’s alleged Oval Office comments ‘undermines trust’ Dems quiz Trump HHS nominee on drug pricing MORE (R-La.), Sen. Mary LandrieuMary LandrieuProject Veritas at risk of losing fundraising license in New York, AG warns You want to recall John McCain? Good luck, it will be impossible CNN producer on new O'Keefe video: Voters are 'stupid,' Trump is 'crazy' MORE’s (D-La.) main Republican opponent, for his support from Koch Brothers-backed groups.

The ad is the group’s second in the state, where Landrieu has already faced negative attacks from the Koch-backed Americans for Prosperity focused on ObamaCare.

“Out of state billionaires, spending millions to rig the system and elect bill Cassidy. Their goal? Another politician bought and paid for,” a narrator says in the ad, as shots of the Kochs flash on the screen.

The ad charges that the Kochs want tax breaks for companies that ship jobs overseas, want to cut Social Security and “end Medicare as we know it,” and “even tried to kill relief for hurricane victims.”

“Cassidy’s billion-dollar backers. They’ve got a plan for him — it’s not good for Louisiana,” the ad closes.

Democrats have launched an extended offensive effort making the Kochs a central focus of their attacks on Republicans.

The billionaire brothers have sunk at least $30 million into competitive House and Senate races since August of last year, largely on ObamaCare attacks. Democrats believe tying Republicans to the Kochs and what they see as their harmful agenda will be politically advantageous as they work to respond to the attacks, even as they expect to be outspent.

Landrieu remains one of the nation’s most vulnerable senators. A survey conducted in mid-February that went public just this week shows Landrieu lagging Cassidy by 4 points among those voters who say they’re likely to back either candidate, and 9 points among those who say they’re definitely backing either candidate.

Landrieu, Cassidy and two other Republicans are sharing the ballot in Louisiana’s all-party primary this November. If no candidate takes more than 50 percent of the vote, the race will head to a runoff.