Charles Koch is firing back at his critics, accusing them of trying to defame him for political purposes.

The billionaire head of Koch Industries, who along with his brother David is a major backer of Americans for Prosperity and other conservative groups that have been hammering Democrats, is up with an op-ed in The Wall Street Journal arguing that his political spending is about free enterprise, not his own bottom line.

"Instead of encouraging free and open debate, collectivists strive to discredit and intimidate opponents. They engage in character assassination. (I should know, as the almost daily target of their attacks.)," Koch writes in the piece. "Such tactics are the antithesis of what is required for a free society — and a telltale sign that the collectivists do not have good answers."

"Rather than try to understand my vision for a free society or accurately report the facts about Koch Industries, our critics would have you believe we're 'un-American' and trying to 'rig the system,' that we're against 'environmental protection' or eager to 'end workplace safety standards,'" he writes before detailing how he believes those criticisms are unfair.

The Kochs' groups have already spent millions attacking Senate and House Democrats this election cycle — Americans for Prosperity has spent far more on the 2014 elections than any other group.

Democrats have sought to turn them into bogeymen heading into the midterms, issuing near-daily attacks on them and their spending, and have begun to try to tie the Republicans they're backing to some of their business practices.