The part of fosteing a weak populist yourself is important, if you don't one of your other characters (usually a prominent one with lots of charisma) will take over the populist party for lack of other males... (just had this happen to me without any proscription campaigns on my part, there simply wasn't any male characters that turned populist for a while and then suddenly a charisma 10 sibbling of my current ruler turned populist, within short he had gotten his party from 4 to 50 seats in the senate and started a civil war).