A lot of the opposition to this law seems based on it being passed by democrats and the belief that it will only benefit them. I don’t think that is the case.
If I vote for a republican in 2020 and a democrat wins Colorado, my vote is effectively turned into a vote for a democrat under the current system. Under the new law (if it is ever implemented), my vote for a republican would actually count in the nationwide popular vote, regardless of how the rest of the people in CO vote. If the republican won the national vote with my assistance, the nine Electoral College votes from my state would count go to the republican, even though the majority of people in Colorado voted for a democrat.
I would certainly like my vote to actually count towards the person I voted for and not who the majority of Coloradans voted for. Republicans are opposing this law, presumably because we have had two republican presidents in the last 20 years who lost the popular vote. I wonder who would be in opposition if we had ended up with two democrats losing the popular vote but ending up in the white house.