It would be better if they rested on their laurels. They don't, they scorn and disdain the very policies they were elected on, because the "base" has nowhere else to go. Instead they grab at the opponent's half of the undecided middle, by implementing the very policies they were elected to oppose. This contemptuous practice is called "triangulation".

NB: in this current presidential race there is at least one Republican who tries to actually shrink government, and two Democrats who actually vote against war. It's rare to find a presidential campaign so blessed with honesty.