Dude is so stubborn.
user
Dude has a chance to leave with grace. If he runs and trump gets elected, he will be remembered the same way RBG is remembered
assistant
In other left subs people talk like with Biden, there's at least a small chance to win, but with other candidate there is none. 

Since I'm not from America, I don't really get, why it is considered " Too late" To build public support for new candidate in half a year. Is campaigning in US so different?
user
&gt; Is campaigning in US so different?

Yes. Unofficially, the next campaign begins immediately after an election. There is never an "off" switch, just times of less intensity. We are bombarded with campaigning in various forms during all 4 years of an election cycle, it's just that it looks slightly more subtle the farther you are away from an election. 

We usually basically know the candidates a year, sometimes more, out from the election. So it is definitely a hindrance if they tried to switch candidates now.
assistant