http://www.sedevacantist.org/savonarola.html

"Cardinal Journet, in his The Church Of The Word Incarnate (Vol. 1, p. 484, trans. A. H. C. Downes, Sheed & Ward 1955) offers the following information about these letters of Savonarola: 'In a study in the "Revue Thomiste" (1900, p. 631, "Lettres de Savonarole aux princes chretiens pour la reunion d'un concile"), P. Hurtaud, O.P., has entered a powerful plea in the case - still open - of the "Piagnoni". He makes reference to the explanation of Roman theologians prior to Cajetan, according to which a Pope who fell into heresy would be deposed "ipso facto": the Council concerned would have only to put on record the fact of heresy and notify the Church that the Pope involved had forfeited his primacy. Savonarola, he says, regarded Alexander VI as having lost his faith. "The Lord, moved to anger by this intolerable corruption, has, for some time past, allowed the Church to be without a pastor. For I bear witness in the name of God that this Alexander VI is in no way Pope and cannot be. For quite apart from the execrable crime of simony, by which he got possession of the [papal] tiara through a sacrilegious bargaining, and by which every day he puts up to auction and knocks down to the highest bidder ecclesiastical benefices, and quite apart from his other vices - well-known to all - which I will pass over in silence, this I declare in the first place and affirm it with all certitude, that the man is not a Christian, he does not even believe any longer that there is a God; he goes beyond the final limits of infidelity and impiety" (Letter to the Emperor). [Footnote : These were neither new nor isolated accusations. cf. Schnitzer, "Savonarola", Italian translation by E. Rutili, Milan, 1931, vol. ii, p. 303.]

'Basing our argument on the doctrinal authorities which Cajetan was soon to invoke, we should say that Savonarola wished to collect together the Council, not because, like the Gallicans, he placed a Council above the Pope (the Letters to the Princes are legally and doctrinally unimpeachable), but so that the Council, before which he would prove his accusation, should declare the heresy of Alexander VI in his status as a private individual. P. Hurtaud concludes: "Savonarola's acts and words - and most of his words are acts - should be examined in detail. Each of his words should be carefully weighed and none of the circuмstances of his actions should be lost sight of. For the friar is a master of doctrine; he does not only know it but he lives it too. In his conduct nothing is left to chance or the mood of the moment. He has a theological or legal principle as the motive power in each one of his decisions. He should not be judged by general laws, for his guides are principles of an exceptional order - though I do not mean by this that he placed himself above or outside the common law. The rules he invokes are admitted by the best Doctors of the Church; there is nothing exceptional in them save the circuмstances which make them lawful, and condition their application."'

Now, even though Alexander VI was a valid Pope, and it is certainly possible that he was a secret and internal heretic, nevertheless, the Precedent is plain: There have been good and holy People in the Church in the Past who have called into doubt the legitimacy of the Pope on the basis of personal heresy. THese people, were for all intents and purposes sedevacantists: They said there is NO POPE because of heresy.

Precedent.