Both "rival" parties represent the same interests of the same small elite group on the top of your society. The current president and current party are like the cloth to them - as soon as this cloth gets too dirty and stinks (if they spent the hell of the time and that was too loud, so that the middle class is inevitably ready to react), they simply change the garment for the new - in reality old washed one.
It's well seen for instance as everybody likes to associate the Iraq gamble and failure directly to the personal adventurism of Bush as if it is Bush who like the great emperors of the past designed the doctrine and waged the nation after the flight of his mind. That is how it works. Now Bush and some small "sacrifice" part of Republicans are drenched in the sins, they like sponge absorbed all the mischiefs and claims and are now ready for the garbage bin.
There is only one party - very small, non-political and not loving publicity - and you always vote for it, though I don't think they represent anything except for their own interests.