Mad Madge, whose latest single is in support of Barack Obama, isn’t the only one injecting politics in to pop music. Vivienne Westwood is tossing her hat into the ring too: “I’ve written a political song about the legalized, 42-day detention of suspected terrorists,” said the designer at the London launch of her compilation CD at Selfridges on Monday. “I never listen to the radio, but I had to recently to see who’s out there to sing it. I didn’t like any of them! So it looks like I’ll have to. I’m not a singer really, but I’ll do something.” Westwood’s CD “Catwalk Breakdown,” a selection of favorite tracks by everyone from Tchaikovsky, The New York Dolls and Luca Mainardi, is on sale this week in Selfridges’ in-house concept store the Wonder Room. Maybe she can get Johnny Rotten on backing vocals.