"Most of the soures" don't have to. TR didn't have to. Merely using it to identify a form of corruption establishes it, especially if the author is the POTUS viewing the corruption from the inside. The word "conspiracy" is not used here. Converging interests of a powerful few suffices to produce the same effect. Nothing is said here about campaign finance, although that is understood. There must be much about campaign finance that is illegal or secret, otherwise people wouldn't keep going to prison, but "illegal or secret" isn't the requirement for corruption. Merely self-serving and dishonest will do, all the more if on a massive scale.