An advertisement attacking GOP mega-donors Charles and David Koch and Rep. Bill Cassidy, a Republican candidate for Senate, is staying on the air in Louisiana despite the threat of legal action from the National Republican Senatorial Committee, after television stations deemed the ad accurate.

The Senate Majority PAC ad, which has aired or will air across each of Louisiana's seven media markets, claims "the out-of-state billionaire Koch brothers funded the fight to let flood insurance premiums soar, helping the insurance companies, and cut off hurricane relief for Louisiana families."

"Now, they're spending millions to buy a Senate seat for Bill Cassidy, so he can fight for them," a male narrator continues in the ad. "If the Kochs and Cassidy win, Louisiana loses."

Republicans balked at that association. In a letter to 34 television affiliates airing the ad last week, the NRSC said the ad is "patently false and misleading" and demanded the stations "stop airing this advertisement immediately."

"If you are unwilling to do so, we reserve the right to pursue any appropriate legal action and request an explanation of the basis of your decision in law or station policy," the letter, penned by NRSC general counsel Megan Sowards, continued.

The threat, as it turns out, was empty.

"Despite the best efforts of the NRSC to defend the Koch Brothers’ anti-Louisiana agenda, our ad remains up in every TV market in Louisiana and on every station where we placed our ads explaining why the Koch Brothers and Bill Cassidy are bad for Louisiana," Senate Majority PAC spokesperson Ty Matsdorf said Thursday.

General managers for multiple Louisiana network affiliates confirmed the same to the Washington Examiner, saying their lawyers had reviewed Senate Majority PAC's supporting documents for the ad and found no legal problems with the information contained therein.

By law, campaign ads produced by candidates are allowed to say just about anything — but content of third-party ads must be supported by fact, lest television stations be held liable for airing them.

The Senate Majority PAC ad's claim is technically correct: The Koch brothers founded and fund Americans For Prosperity, a pro-Republican outside group that has aired millions of dollars-worth of attack ads against Cassidy's Democratic opponent, incumbent Sen. Mary Landrieu. AFP also opposed flood insurance and Sandy relief legislation in Congress this year.

What the ad doesn't mention is that Cassidy co-sponsored the flood insurance measure — an oversight with which the NRSC took issue.

Although this ad challenge fell flat, NRSC legal counsel has warned the Louisiana television stations that they will hold this line moving forward, according to committee sources.