Swing states will never ratify it so you need some red states to ratify it.
What if Dems conclude that the electoral college puts them at a structural disadvantage (based on, among other things, the 2016 result)? It would then be a partisan issue in swing states as well as safe states, even though the state as a whole would be seen as a potential loser. Then couldn't you imagine a scenario where at least some swing states pass it at some point in the next ~10-15 years, at some point when the Dems are riding high and have control of both houses of the legislature in more than a few swing states?
That could still leave you a few states short, but there's also the possibility of it passing by referendum in a handful of states necessary to put it over the top. No, all of this isn't going to happen by 2020, but could it happen by the 2030s? I think it's entirely possible.
