Election Day always follows hard on Oct. 31, so political ads regularly riff on and rip off Halloween, erecting gratuitous gravestones or mining images from scenes in gory horror flicks. In a current anti-union Web ad, Al Franken’s head appears delighted enough to dance disembodied on democracy’s grave. And a new Republican mailer Photoshops U.S. Rep. Betty McCollum’s face into that of a chainsaw killer. Yet it’s a holiday hook that’s not without its hazards — as a business group learned in Minnesota six years ago, with a mailer that seemed merely macabre one day but appeared to mock the dead the next.