To add onto the answer below, it is also like a Strategic Base Location in the Military. Even if it remains unused, others will know of it's existence, and it can influence the way they do things. Political Capital, in the Strategic sense of things is also very much like Social Capital. An investment or favor now may produce future benefits, either financially or positionally. For example, if I go to dinner with the top party leaders, and provide some added treat, or arrange for a favorite band at a fellow Senator's Birthday party, or get his son/daughter a coveted summer intern position, or vote for his/her unpopular earmark bill... Later, when I need a favor, or some votes, or so forth, they will find their way to me, with just a word in the right ear.

     This is as corrupt as a cop on the streets taking bribes to look the other way, while financing his kid's college education, or the Umpire who tilts a game in favor of one team just cause they sent him a few boxes of Cuban Cigars and a new Car.

BUT, it is also one of the things that no single politician from either major party, nor the minor parties can fix or change by themselves. It would takr the reversion of Moral Responsibility back to 1940's or 1950's levels.