It’s my impression that it’s mainly the “center”-left parties (aka the larger ones) which got infiltrated, especially in countries with power duopolies thanks to Mathematically rigged voting systems like First Past The Post.

If you think about it, it makes total sense that Greedy Posh Salesmen-types with no principles whatsoever who want to get rich quick would be drawn to the two parties in power duopoly systems, were they will sell themselves to those who can reward them the best for their services, and those people certainly aren’t the Poor or the Working Class.

I’ve concluded some years ago that the secret for Democracy to actually do what it says on the tin is constant change (the very opposite of those voting systems mathematically rigged for “stability”) - rotten politicians must genuinely fear that if they sell themselves, they WILL end up out of power and the next ones in aren’t going to be just “mates with a different pin on their lapel” so they are more likely than not to end up in jail for it.