National Popular Vote

Electoral votes = House seats + Senate seats. House seats are proportional to population. Senate seats are constant. What this means is that Wyoming, with 3 votes, has 180000 people represented in each vote, while California has 670000 people per vote.

Also swing states would be unwilling to enact it on an individual basis, from a game theory perspective. Take a swing state worth 9 votes, convert it to a proportional system. Suddenly instead of 9/0 vs 0/9 you have 5/4 vs 4/5, and it's now really worth only 1 vote.

It's a famous political science question in Japan as well ("一票の格差" or disproportionate representation). According to the Wikipedia page on this subject (which is only in Japanese for some reason) most countries force redrawing of electoral districts when one vote is worth much more than another. In Japan it's 3 times value for lower house and 6 times value for the upper; Britain is about 1, France 1.5, Germany 2 and Italy 1.22 (though the limit is broken sometimes). Some countries like the Netherlands and Israel has no electoral districts at all.
The United States House of Representatives is below 1.4 times but the Senate is over 70 times. Quite horrendous compared to other developed nations

The whole issue of state sovereignty is at hand, although Great Britain which has four parts have very low (almost no) disparity in the value of one vote. Votes in swing districts/states would also have a greater effect on the outcome of the election while votes in overly liberal/conservative/etc districts wouldn't "count" at all. The only fair way to do it is the Netherland/Israel way of having one national district and all votes going towards that one vote count. Would go a long way towards getting rid of pork-barrel spending (but would encourage demagogues/celebrities from running).