In truth we have no way of knowing how much bribery, blackmail, and espionage are occurring right now. But it would be unwise to forget that these clandestine practices have always gone on under the public's nose, no matter how many laws are passed and penalties brandished against them. We are fated to be governed by one-eyed jacks. Sometimes corruption thrives openly, because it isn't widely recognized as corruption. Buying votes with government money is now accepted practice in democracies, though it is merely a generalized form of bribery in which politicians bribe voters.
Voters can have conflicts of interest as surely as candidates can.
The inevitability of corruption is one of the strongest arguments for limited government. A dishonest private merchant hurts only those who do business with him. A corrupt government official hurts everyone, because everyone is forced to do business with the government.