The AI cheats. When the game first came out, I tried to lead a push to reduce or eliminate the cheating but apparently they decided to keep it in. Its a shame, because it makes playing on the higher levels pointless (its all about California at that point).
But this is a bug. Sometimes the AI uses political capital and sometimes it doesn't. Lately I'm having trouble getting it to use it, but I did run a campaign against Vilsack where he used it. Not well, but he did at least use it.