According to this report,
The liberal group Media Matters has quietly transformed itself in preparation for what its founder, David Brock, described in an interview as an all-out campaign of “guerrilla warfare and sabotage” aimed at the Fox News Channel….Media Matters, Brock said, is assembling opposition research files not only on Fox’s top executives but on a series of midlevel officials. It has hired an activist who has led a successful campaign to press advertisers to avoid Glenn Beck’s show.
If their opponents are spreading false or misleading arguments, couldn’t “progressives” disprove those arguments? If progressives have a sound case to make, why would these thuggish tactics be necessary?