To be honest -
If Obama were eligible to run AGAIN - I do think we'd see rallies of comparable size. Remember, he DID have rallies like this, in 2008.
I don't know a Democratic President since, say, Kennedy that retains such loyalty and has that kind of star power among the base. The reason we don't see Bill Clinton doing this - well, besides the fact that he is now mega old and out of shape - is because he can't get elected again - and his Presidency is a little tarnished at this point.
LOSING candidates? Well, there are only so many still alive. McCain and Dole are gone. I guess that leaves Gore, Hillary (both of whom STILL MAINTAIN THEY WON AND WERE CHEATED, by the way) and Romney. Well no one likes them that much, although Hillary is still hinting she'll run again, even after having said she won't. I really wish they'd reach back in the well for her. It would be great to see her LOSE AGAIN - to anyone.