This is an astonishing assertion, made more astonishing by its abruptness and the paucity of explanation. Menashe was perhaps the most reviled Jewish king in the Tanach. Rav Ashi was one of the greatest Amoraim; he was the long-time head of the Sura academy and was one of the two principal editors of the Gemara. Yet he is told flatly and firmly that, had he been in different circumstances, he would have succumbed to Menashe's charisma and joined in, enthusiastically, the Bible's number one bad practice, idolatry.
This midrash resonated when I read a story today about collaboration (or alleged collaboration) by celebrated authors with totalitarian governments -- Gunter Grass, Ignazio Silone, and, alleged most recently, Milan Kundera. (See