The assumption that elected officials had the best interest of the country in mind is key for the Constitution to work, and that's obviously not the case anymore.

I'unno about that -- call it pedantry, but the whole point of the checks and balances installed within the US government is to minimize the damage bad faith agents can do, and there are tons of documents from the founding days that specifically highlight the potential risk that malevolent officials can cause, most prominently so in Washington's Farewell Address, where he explicitly warns that hostile forces will seek to exploit the open nature of the american experiment by infiltrating it. The founding fathers were clearly aware of the possibility, and actively tried to take measures to protect the system.