A commercial critiquing CNN’s Lou Dobbs is being shown on Fox News and MSNBC this week — but not CNN.

CNN has worked with the cable operators that carry its channel to block the commercial, which was produced by the liberal media watchdog group Media Matters. According to a CNN employee who requested anonymity, CNN managers said in a morning staff meeting that the channel had invoked unspecified agreements with operators to stop the ad from running.

The ad accuses Mr. Dobbs of “promoting the false, right-wing conspiracy that President Obama hasn’t produced a valid U.S. birth certificate.”

It calls on viewers to “let CNN know there’s nothing ‘legitimate’ about racially charged paranoia.”

Media Matters sought to buy air time on the cable news channels in Washington, New York and Atlanta this week.

In a statement, CNN said that it “retains the right to object to any ad run by the cable operator on our network whose purpose is to attack CNN or our employees.” A CNN spokeswoman said she didn’t know if the word “agreement” was used in the staff meeting.

WhoRunsGov first reported that CNN had “privately pressed cable operators not to run” the ad.

Eric Burns, the president of Media Matters, said that CNN’s blocking of the ad “seems like the actions of a network desperate to provide cover for its primetime host rather than living up to its standard of being the most trusted name in news.”