Each state can already do this anytime they want; the individual states determine how their EV's are awarded not the federal government.
It's just not that simple.
True, each state determines how they will choose electors. But the fact that each of 48 states use the inane "winner take all" system is
directly dependent on the fact that the other 47 are doing it. That's how we got to this nefarious point. No individual state is likely to give up on that if its neighbors are still doing it --- so it's a stalemate. A product of mob mentality. So it's a system that locks itself in; it either has to change collectively --- or it won't change at all and we'll stay stuck with it.
So, if you want this system put in place for your state go start lobbying your state legislature (unless of course you live in Maine, in which case you already have that system).
Actually no you don't. Maine and Nebraska are the only exceptions to statewide "winner take all" but in their case they still use WTA on the Congressional District level -- which is virtually just as bad as doing it statewide.
As just laid out, lobbying an individual state legislature is pointless, unless you can also lobby other states to do it at the same time. It's the mob-rule mentality, and it's why this hasn't ever been fixed.
This is up to each state political party committee to determine, some states already do it this way, some don't.
No state does it this way. Everybody does WTA, 48 on a statewide level and 2 on a disctrict level. Nobody awards proportionate to actual voters' wishes.