This depends greatly on what kind of political espionage you are talking about.
If you are talking about international political espionage, then my opinion is that it is a legitimate activity that all countries will engage in if they are capable of doing so. Our world is so interconnected that it can be important for decisionmakers in one country to know what is going on in the politics of another.
On the other hand, covert espionage against political opponents in one's own country is not acceptable. Doing something like President Nixon's people were trying to do in the Watergate break-in is simply unethical. In domestic politics, the goal should be to win fairly, not to win by any means (whether it be spying on your competition or intimidating voters) necessary.