Early studiers of cults were surprised to discover than when cults receive a major shock—a prophecy fails to come true, a moral flaw of the founder is revealed—they often come back stronger than before, with increased belief and fanaticism. The Jehovah's Witnesses placed Armageddon in 1975, based on Biblical calculations; 1975 has come and passed. The Unarian cult, still going strong today, survived the nonappearance of an intergalactic spacefleet on September 27, 1975. (The Wikipedia article on Unarianism mentions a failed prophecy in 2001, but makes no mention of the earlier failure in 1975, interestingly enough.)
Why would a group belief become stronger after encountering crushing counterevidence?The shock to the Democratic Party was Trump winning the 2016 election.
The conventional interpretation of this phenomenon is based on cognitive dissonance. When people have taken "irrevocable" actions in the service of a belief—given away all their property in anticipation of the saucers landing—they cannot possibly admit they were mistaken. The challenge to their belief presents an immense cognitive dissonance; they must find reinforcing thoughts to counter the shock, and so become more fanatical. In this interpretation, the increased group fanaticism is the result of increased individual fanaticism.
In Festinger's classic "When Prophecy Fails", one of the cult members walked out the door immediately after the flying saucer failed to land. Who gets fed up and leaves first? An average cult member? Or a relatively more skeptical member, who previously might have been acting as a voice of moderation, a brake on the more fanatic members?
This is one reason why it's important to be prejudiced in favor of tolerating dissent. Wait until substantially after it seems to you justified in ejecting a member from the group, before actually ejecting. If you get rid of the old outliers, the group position will shift, and someone else will become the oddball. If you eject them too, you're well on the way to becoming a Bose-Einstein condensate and, er, exploding.