All of a sudden. "standing" will be a forgotten term.
If the Dems are stupid, it will be after the election (making the VP elect the President?)
If they are not quite as stupid, they will wait until he appears to be the nominee.
I’d guess the SC would defer to the 50 Secretaries of State, the Electoral College, and Congressional certification of the election. The SC doesn’t want a “penumbra” here and the Constitutional language isn’t clear enough for interference.