The way I understand it, individual states would vote to change their election laws to award their electoral votes to the winner of the national popular vote. Legal as can be and does not touch The Constitution.
Unless they have actually done that, the claim is bullshit. Any state that does do it is just diluting its influence on the national election. It's not in the best interests of any state to do it. Therefore, it will never happen.
Well it's perfectly legal and constitutional. Every state has the right to determine for themselves how they designate their delegates. But if you go by popular vote the power will go to major cities and that favors the Democrats...hence the reason why they favor it. If you go proportional by counties or congressional districts the Democrats will never win a presidential election again....hence the reason why they oppose that.
The simple fact though is that what is good for a city is not always good for rural America. What is good for Florida is not always good for Kansas. What is good for California is not always good for Idaho. The system was designed to balance everything and ensure that everyone's interests have influence but don't have dominance. Is it perfect? No. I live in Oregon and can attest to that. The entire state is extremely conservative except for Portland and Eugene which are ridiculously liberal....but those two cities dominate state politics and Oregon's influence in federal government because of their population compared to the rest of the state.
So no it's not perfect but it's better than the alternative of allowing states with major cities to control policy at the expense of rural America