A quibble: I can’t think of any that have resigned under pressure, but many have simply voluntarily packed up their bags and moved on. Secretary of Interior Ken Salazar comes to mind immediately as just one example.
Ah, I rummaged around a little and see that the AG is indeed part of the Cabinet. I thought is was just “Secretary of X”. In that case, AG’s Mitchell (Nixon) and Meese (Reagan) did indeed resign under pressure.
I sure that some might have resigned under pressure, but I am unaware of any of them that have stood up and said I don’t agree with what the President is doing and therefore I resign. This was the case with William Jennings Bryan.
In this country, there are very few who stand up and state I don’t agree and I hereby resign; or I should be held responsible for actions that occurred during my watch.
When was the last time a military officer tendered his resignation? Looking at the mix of officers at the upper command echelons, I fear for this country.