the problem is that the npvic is only binding after it receives a number of signatories with over 270 electoral votes, as     notes but if, say, a pile of states were to sign onto the npvic before the election is over, yet doing while trump has a solid unbeatable popular vote lead, they could perhaps all get shanghai'd into doing so, after which the late joining election flipping states simply leave the pact and the npvic probably collapses to prevent this from ever happening again as everyone behind it gets lynched by their own
United States