Anything to shake up politics. Also, it would be nice to have a candidate who actually kept his campaign promises once he got into office, and I think the Devil would, for the sake of garnering as much popularity as possible.
Of course, if the Devil actually existed, I doubt he would allow himself to get lured into anything as pointless as a war with God. After all, he is likely to be many orders of magnitude more intelligent than you or I, and even we little mortals know there's no way you could win such a war. It would be more within the Devil's interest to just play ball with God and make his own moves on the side.
The Rev