This isn't a provision I've studied. FindLaw's Annotated Constitution (which shows how the courts have "amended" the Constitution over the years) says only this about that section:

At the time of the Civil War, this clause was one of the provisions upon which the Court relied in holding that the Confederation formed by the seceding States could not be recognized as having any legal existence. 1797 Today, its practical significance lies in the limitations which it implies upon the power of the States to deal with matters having a bearing upon international relations. In the early case of Holmes v. Jennison, 1798 Chief Justice Taney invoked it as a reason for holding that a State had no power to deliver up a fugitive from justice to a foreign State. Recently, the kindred idea that the responsibility for the conduct of foreign relations rests exclusively with the Federal Government prompted the Court to hold that, since the oil under the three mile marginal belt along the California coast might well become the subject of international dispute and since the ocean, including this three mile belt, is of vital consequence to the nation in its desire to engage in commerce and to live in peace with the world, the Federal Government has paramount rights in and power over that belt, including full dominion over the resources of the soil under the water area. 1799 In Skiriotes v. Florida, 1800 the Court, on the other hand, ruled that this clause did not disable Florida from regulating the manner in which its own citizens may engage in sponge fishing outside its territorial waters. Speaking for a unanimous Court, Chief Justice Hughes declared; ''When its action does not conflict with federal legislation, the sovereign authority of the State over the conduct of its citizens upon the high seas is analogous to the sovereign authority of the United States over its citizens in like circumstances.'' 1801

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[Footnote 1797] Williams v. Bruffy, 96 U.S. 176, 183 (1878).

[Footnote 1798] 39 U.S. (14 Pet.) 540 (1840).

[Footnote 1799] United States v. California, 332 U.S. 19 (1947).

[Footnote 1800] 313 U.S. 69 (1941).

[Footnote 1801] Id., 78-79.

I know states enter into agreements with one another all the time, such as Alabama-Florida-Georgia over river use and Virginia-DC-Maryland over various transportation issues. I don't know whether they have to be ratified by Congress.

Copied from comments to subsequent post here.

No sh%t Sherlock. It'll be like the 2000 elction - they'll just want proportional voting in Red states and they'll want to be able to change it after the votes come in.

How can this be constitutional? I don't dispute that States can allocate their votes in whatever way they choose, on their own, but this deal requires a compact among States, and Congress must approve it:

No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.

Article 1, Section 10 of the constitution. The pact among the States would be unconstitutional until Congress approves.

California, for instance, doesn't count their absentee ballots is the margin in the general election exceeds the number of absentee ballots. In an winner takes all electoral system, this makes sense, since the counting would be wasted effort. If other states are counting on CA's count as part of a 'national popular vote count' though, this becomes important.

As an example, in 2000 CA left nearly 2 million absentee ballots uncounted. Since Gore won CA by nearly 4 million votes, this didn't affect the election results. It is, however, much largely than Gore's national margin, so the common assertion that Gore won the popular vote could be completely wrong.

Now suppose Ohio was, in 2000, using this proposed national popular vote proposal and California was not. They have no way of making CA count the extra votes. They have no way of determining the actual popular vote count. So how do they determine which way to vote?

Thanks to the electoral college, though, fraud in such places doesn't effect the election outcome. Since the party has already won that state's electoral votes, no number of fraudulent votes can help the candidate get more.

The only place where fraud will change the outcome is in states where the parties are evenly divided and fraudulent votes could flip the state from one party to the other. And that is precisely the states were it is hardest for one party to get away with something without the other party finding out and quashing it.

If we switched to a pure popular votes, strongly partisan states would get into a race to see which could stuff the 'national ballot box' the most for their particular side.