I agree with you. It is about corruption, but I see the thrust as to seduce the viewers into rooting for the bad guys, and to the extent that you can find even one principled person portrayed, that character will end up being one of the losers. The show screams that this is reality, and don't expect anything else. Yet, it isn't voiced as a complaint. The writers want us to embrace the ugly power grabbing as the given, and just choose which corrupt side to back. It is an accurate portrayal of Washington, but a dark and biased view of human nature, and purposefully leaving out any image of people of character or principle who win, or even have a shot at winning.
The shows deeper theme is that there are no principles... and if someone acts as if there are, it will make them a fool who will fail and that that is the way it will always be, perhaps even should be.
On the technical level of writing, acting, directing and filming is extremely well done. And it one of the really sad things about trends in our culture that some of very best writers, and the big money, go to polished production of anti-value pieces. This is what comes of generations of anti-life, anti-heroic values being taught in the univerities... the best and brightest come out a polished purveyors of bad philosophy in the arts.