Tell him you understand his pain and self-loathing. Tell him you understand his shame at having supported Willie, the lying scumbag and lying time and time again to support him. Tell him that it is natural for him to take that self-hatred and point it at some innocemt (in this case, Bush) in an effort to relieve his own pain.
Tell him that you know he would aim the blind self-disgust as anyone else and if it wasn't Bush, it might be another relative, his boss, a baseball player, an advisor (like a broker or tax preparer). Tell him that he should not be concerned about his foolish stupidity and should understand that it is irrational and without factual support. Tell him that, despite all of this, you forgive him. He may be too stupid to understand forgiveness but at least it will piss him off.