Rachel Maddow spent a lot of time Thursday night on what is potentially the most damaging Herman Cain scandal: charges that his campaign manager, Smokin’ Mark Block, used corporate money donated to Prosperity USA to pay Cain campaign expenses. Prosperity USA is affiliated with Americans for Prosperity, the conservative corporate advocacy group funded by Charles and David Koch.

Cain, of course, strenuously denies the allegations of sexual harassment against him, but when it comes to charges that he’s too close to the rapacious Kochs, he pleads guilty. “I am the Koch brothers’ brother from another mother,” Cain told an Americans for Prosperity summit Friday, insisting he was “very proud” to know the right-wing Kochs, who run oil and gas behemoth Koch Industries.

But now Citizens for Responsibility and Ethics is asking for a probe of the ties between Americans for Prosperity and the Cain campaign. The Milwaukee Journal-Sentinel reported this week that Block used Prosperity USA funds to pay for Cain’s travel and other campaign expenses. Block headed Prosperity USA before he joined Cain, and before that he was with Americans for Prosperity. Back before that, of course, he ran a Wisconsin judicial campaign that went so afoul of campaign law Block found himself barred from politics in Wisconsin for three years.