I guess Standard Oil was one of the early big cases . Ironically, the companies that rose from its ashes became part of another cartel (the seven sisters) afterwards. I understand that companies need to get bigger in order to get economies of scale, but, at some point, if they become too big, like ogres, they end up bullying entire sectors.
I guess that the political party funding in the US doesn't make it easy... politicians owe debts (real or implicit ones) after the elections, don't they?
I guess that the struggles between the political and financial power is an old story. The most famous case we have here is between Louis XIV and his Superintendent of Finances. At that time, the politicial power (the monarchy) was much stronger, because it had traditions (and the army) to back it up.