Harris may poll better against Trump, but it doesn’t matter if Heritage can ensure she isn’t on the ballot in states where the filing deadline is passed.
Not the direct point, but Harris is on the ballot anyway, so even if Biden withdrew a week before the election (which to be very clear: he should not) she'd automatically inherit his campaign.
It's one of the things the states burned in when they built up their respective faithless elector laws: if the guy at the top of the ticket is there, you have to cast your EC vote for them; if they are not, then for their VPE, and if neither, then some secret third thing (ie. differs state-to-state)
that we haven't nationally just formalized it so it works as everyone thinks it should straightforwardly rather than works 95% like everyone thinks it does by some gigantic collection of interlocking and imperfect cogs in a giant conlaw rube goldberg machine is completely insane, but here we are
(bell curve meme)
dumb: the president is elected by people voting for president and him getting to 270
US constitution: noooo noo actually they're elected by the EC electors, and the state legislatures get to
Jedi: the president is elected by people voting for president and him getting to 270
It's not clear that it even requires an amendment. If states controlling a majority of the Electoral vote promise to pledge their electors to reflect the national popular vote, even if that differs from the popular vote in-state, the rest don't matter. An active effort is 77% of the way.