To Paul Midford:
"Fukuda, Abe,
and Mori for that matter, as far as I can tell, resigned of their own volition, not because they were forced to. Of course, that's different from saying the PM's popularity does not matter, or that knowledge of the importance of the PM's popularity did not weigh on these leaders' decisions to resign. Kan might be the single example where an actual revolt did endanger his hold on power, although I am not sure whether he would have had to if he had called Ozawa and Hatoyama's bluff. "

ESK: No Paul, Mori was forced out (fear that the party would do disastrously in the next HOC election), and Abe was under great pressure to resign from the party until he finally fell on his own sword and checked himself into a hospital and resigned. It is quite common for parties in parliamentary democracies to put pressure on ineffectual or unpopular leaders to resign.
The party may not vote them out or "force" them out publicly which seems to be the standard you are using, but most will take the face-saving gesture of resigning when they are under enough pressure to "take one for the team." Paul Rudd the Australian PM was ousted in an intra-party coup in 2010. Milliband, leader of Labour in UK, is under some pressure now from within his party. Whether they actually do resign or not depends on how bad the situation for the party is and whether the leader can convince his party he can do better.
Best regards,
Ellis

Ellis S. Krauss
Professor,
Graduate School of International Relations and Pacific Studies University of California, San Diego La Jolla, CA