There's a substantial difference between this scandal and every other.
The Clintons secured the nomination using every possible dirty trick. The Democrats were less than thrilled by a Hillary candidacy, but they assumed that the risk was low. Scandals could be managed by the media.
If however Hillary has a major medical condition, then she will have betrayed the Democratic Party, putting the election at risk by securing the nomination under false pretenses. Such a revelation would be explosive.
This isn't a scandal anymore. It's a circular firing squad.
Hillary as a weak candidate with scandals is one thing. The Dems saw that coming. Hillary staggering through an election while dealing with serious medical problems will be a very nasty surprise. It will mean that she knew she was unfit, but forced the nomination anyway.
The Clintons have hurt the Dems in the past, but this is something of a different order entirely. It isn't just a scandal. It's not an immoral act or criminal activity. The Clintons have had plenty of those. This breaks faith in a very basic way with the party. It exposes the fact that the Clintons will not only do anything to win, but that they will casually put their party and its future at risk to do so.