It's been a totally radical day for Sen. Harry Reid.

So "radical," in fact, that the Senate majority leader used the word 21 times in a floor speech Thursday morning, to criticize the billionaire conservative donors Charles and David Koch.

Reid has undertaken an anti-Koch crusade of late, blasting the brothers for funding ads that attack Democratic candidates. Last month, Reid called the brothers "un-American."

Profiled by The New Yorker's Jane Mayer in 2010, the billionaire oil magnates have funded attack ads and political organizing against Democratic candidates through the group Americans for Prosperity, a 501c4 group that does not disclose many of its donors or activities. Thanks in part to Democrats' efforts to criticize them, they've become icons of secretive campaign money-although, given that 501c4 money is indeed secretive, no one really knows how much the Kochs, or other top donors on either side of the aisle, actually funnel toward attacks each election cycle.

Campaign finance, and secret donations in general, are under a political microscope this year as the IRS considers a new rule that would define political activity and potentially force 501c4 groups, which do not disclose their donors or how much they spend on "issue" ads, to eventually reclassify themselves and begin reporting their activities to the Federal Election Commission.

Democrats have argued for donor disclosures, while Republicans have argued that donations, as a form of speech, should be allowed to remain anonymous in many cases, citing the IRS's targeting of tea-party groups as a harbinger of government suppression of conservative views.

Reid and other Democrats have sought to make the Kochs poster children for "shadowy" campaign money. Republicans have protested that it's inappropriate for Reid and others to single out individual citizens and criticize them for their political views, calling the attacks by Reid and others suppressive and intimidating.

Reid seems to feel strongly that he's in the right. His prepared floor speech only included 12 uses of "radical," but he nearly doubled that count, apparently on the fly.

"When Senate Republicans defend the Koch brothers, they are also defending the Koch brothers' radical philosophy," Reid said, going on to ask whether Republicans would defend the "radical" agenda to roll back environmental regulations, repeal Democratic health reforms, and alter Social Security and Medicare, repeating his question as a refrain throughout the speech.

"Those were Charles Koch's own words, radical philosophy, by the way," Reid said, referencing Charles Koch's insistence, to Reason's Brian Doherty, that libertarianism must be a "radical" movement.

"Because we have a radical philosophy, we don't appeal to people who are in positions of influence, people with status or wealth," Koch told Doherty for his 2008 book "Radicals for Capitalism: A Freewheeling History of the Modern Libertarian Movement."