A sad result of the "Politicians are all crooks!" rhetoric which has been around for decades is, apparently, that it means that when there's an actual crook running for office, people don't think it means anything.
Though I'm sure it goes back further, I would guess that the trope got revivified around the time of Watergate–as a way of minimizing Nixon's crookedness.
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@DavidSalo and somehow the government is the enemy got turned into only half of it is! And it’s the one not in power!