In its base form, comedy is about contradictions, whether it's an Adam Sandler movie about a rough-and-tumble misfit in a refined environment or a Tom Stoppard play about lost Shakespearean characters. When comedy turns political, as is the case with comedian Barry Crimmins, that contradiction takes the form of dissent. The job of the political comedian is to question consensus and common wisdom. Crimmins has been doing that since the Reagan administration, taking on subjects from the Iran-Contra hearings to Enron and Afghanistan.

''Even mainstream humor trades in dissent, whether it be about pop culture, family life, or any number of subjects considered by comedians,'' says Crimmins. ''Sassing back at things that encroach upon our lives is funny. It provides relief and refutation. Different answers to mind-numbing and predictable nonsense make us laugh.''

About 10 years ago, Crimmins found a partner in dissent when he met controversial historian Howard Zinn, the subject of a documentary for which Crimmins will host a fund-raiser Sunday at the Green Street Grill. The two crossed paths in the '80s, speaking at the same rallies and attending each other's lectures and shows. For his part, Zinn was thrilled to see a comedian with a knowledge of history and politics.

''I had seen him perform and was knocked out by his combination of comedy and politics, which I hadn't seen before anywhere,'' says Zinn. ''That is, not since I was watching Dick Gregory or, you know, that generation. But in this generation, he was the first comic I'd run into who had political intelligence.''

Zinn's influence on Crimmins stretches back to the '70s, when Crimmins's sister returned from studying at Boston University, where Zinn is professor emeritus, and turned her younger brother on to Zinn's work. Later, while Crimmins was helping to create Boston's '80s comedy boom at the Ding Ho club in Inman Square, he was reading Zinn's books, including ''A People's History of the United States,'' a landmark work in the alternative history movement.

Neither can recall exactly when they started talking on a regular basis, but they've been friends for more than 10 years. Though Crimmins left Boston in 1994, he and Zinn still get together to drink coffee and talk whenever they're in the same city. Crimmins cites Zinn as a valuable resource for and influence on his comedy.

''When I think of Howard, it's been amazing to me that I can read his stuff and that I actually know him and I can call him up,'' says Crimmins.

Crimmins credits Zinn with the ability to make sense of esoteric political ideas in a way that encourages people to dig a little deeper into the issues. ''In theory, I try to do the same thing with my comedy,'' says Crimmins. ''I try to take a very complex set of world affairs that we deal with and make them accessible to people in a way, and give them things so that they can understand things a little better.''

Zinn believes Crimmins is in a unique position to challenge conventional wisdom through political comedy. ''What distinguishes Barry is that he's bolder than anybody else, politically,'' says Zinn. He believes comedy can be an important vehicle for communicating ideas, albeit an underused one. ''To me, comedy can serve a very powerful social purpose, and it has its own special power that ordinary political rhetoric doesn't have,'' he says.

As for Crimmins, he acknowledges the limitations of art as a means of effecting immediate change but does think stand-up comedians can have an impact on politics and society.

''We can continue to be provocative, you know, ask questions,'' Crimmins says. ''And we can smuggle content to people in the form of pop culture. Just the same as pop culture is used all the time to smuggle other ideas to people, like buy this product, look like this, be like this, go along with this. You can do the same thing and present other ideas.''