"Left-wingers tend to like cartoonist Aaron McGruder for his comic-strip assaults on the Bush administration in "The Boondocks." But at a dinner Sunday night marking the 138th anniversary of the Nation magazine, McGruder earned boos for saying liberals need to be meaner if they want to win back the White House.

The cartoonist revealed that he voted in 2000 for spoiler candidate Ralph Nader, and also had the nerve to blast Al Gore for losing. "Noble failure is not acceptable," he told The Post's Richard Leiby, recounting his remarks yesterday. "You've got to be prepared to get your hands dirty. You need to do whatever it takes."

An unscripted reference to national security adviser Condoleezza Rice -- whose love life has been the topic of McGruder's strip -- also caused some discomfort in the audience, which included actress Uma Thurman, director John Waters and author E.L. Doctorow.

"I've met Condoleezza Rice and called her a murderer to her face," McGruder said. From his studio, the cartoonist opined, "This is what I do. I am always going to speak honestly. I'm always winging it, and that's one of the things that gets me into trouble."