Microsoft and Apple are waging the most political ad war ever.

There has always been a smug vitriol coursing through Apple’s “I’m a Mac … and I’m a PC” ads. From day one, the point of the campaign has been that both machines are computers—yet they have wildly different expectations of what a computer should do for you. Justin Long, the actor who plays Mac in the ads, wears a pitying curiosity on his face whenever talking to John Hodgman’s PC. The critique is handled deftly—PC’s motivation may rarely be questioned, but his tactics and capabilities are skewered ad nauseum. It’s a political approach to corporate advertising. Chide your opponent without alienating your audience.

When they first started in 2006, Long and Hodgman’s sketch seemed harmless. Microsoft treated the Mac ads like a smear campaign—no need to legitimize it with a response. Apple was Apple—the forever-quirky underdog destined to dwell in the basement of the computer industry. And Microsoft was Microsoft—an apparently impenetrable fortress of market share.

But this year, the walls were breached. Vista’s lackluster launch caught up with Microsoft, and Apple’s brand continued to strengthen behind iPods and iPhones. Tuesday, Apple announced it shipped 2.6 million Mac computers last quarter, its highest total ever and a 21 percent growth from the same quarter last year. Microsoft started feeling the pressure months ago, and decided it was finally time to do something—$300 million worth of something. It started attacking the attack ads.

First, it did so obliquely. To show that all the negative press about Vista—some of which Apple helped create—had taken on a life of its own, Microsoft commissioned the Mojave Experiment. It’s a classic example of taste-test advertising. Microsoft brought unsuspecting computer users into a room, sat them down in front of a computer with Vista, and proceeded to show them all its neat toys. Then Microsoft released videos—see below—of people saying great things about the operating system, only to be shocked when it turns out to be Vista. The message: Liver and onions don’t taste as bad as everybody says they do.