Viacom, which a few weeks ago refused to run MoveOn’s ad about the consequences of Bush’s tax cuts because they deemed it controversial, has again censored an ad. Viacom said the billboard Missouri Democrats wanted to post was “negative” and an “ambush ad.” The GOP, of course, has a long history of attack ads and character slurs dating back to the days of Lee Atwater. Bush’s father ran the infamous Willie Horton ad. Viacom owns CBS, which refused to run the MoveOn ad but runs the Bush administration ad touting the Medicare bill it forced through Congress recently.