Now that the Supreme Court has vouchsafed the power of a state to require its presidential electors to vote in line with their state’s popular vote, a new question glimmers in the constitutional mist: Could a state require its electors to vote against the wishes of the state’s own voters? That might seem a ridiculous question. Feature, though, the National Popular Vote Interstate Compact.

It’s a workaround designed to commit the states to use the Electoral College to deliver the presidency to the winner of the national popular vote. It’s the first thing that came to mind when the Supreme Court today unanimously concluded that states have the power to punish faithless electors. Most justices credited the language in Article 2, which grants states the power to appoint electors.

The key phrase is that each state shall appoint its electors “in such Manner as the Legislature thereof may direct.” The court, in an opinion by Justice Kagan, reckons this gives the states the power to attach conditions to the electors it appoints, such as the requirement that they vote for the candidate their home-state voters prefer. It can punish them if they don’t.

The National Popular Vote Interstate Compact, though, is a scheme under which states agree to instruct their electors to ignore what their own state’s voters want and, instead, vote for the winner of the national popular vote. The compact goes into effect when it has been ratified by states whose combined electoral vote count is 270, i.e., enough to choose a president.

So far, some 15 states, with a total electoral vote of 196, have signed the confounded compact. The Sun is against it on the grounds that an Electoral College that handed up Washington, Adams, Jefferson, Jackson, Lincoln, the Roosevelts, Calvin Coolidge, and Ronald Reagan, among others, ought not to be trifled with. We can see, though, that it’s newsworthy.

Which is why we’ve written about this issue half a dozen or so times in recent years. And why, after today’s ruling, the Wall Street Journal plated its vast array of presses with an editorial marking the issue. For the court’s decision on faithless electors suggests the Nine might well let a state legislature require the state’s electors to ignore the state’s own voters.

Just to mark the point, what the National Popular Vote Interstate Compact would have done, were it in effect in, say, New York in 2004, is require the Empire State’s votes in the Electoral College to be cast for George W. Bush — even though the Texas Republican lost the popular vote in New York State to Senator John Kerry by a whopping 16 percentage points.

The Journal covers one caveat, which is that the Constitution forbids any state from entering any compact or other agreement with any other state absent the approval of Congress. That prohibition, the Journal warns, could spell heavy sledding at the Supreme Court were the National Popular Vote Interstate Compact ever enacted by enough states to put it into effect.

One reason all this has been on the boil in the past generation is that 2000 and 2016 were both years in which the Electoral College chose a president who’d lost the national popular vote — and, in Mr. Trump’s case, by a substantial tally. That may be the constitutional way of doing things, but it’s not surprising it produces a certain amount of sturm and drang.

So why not simply amend the Constitution to provide for electing presidents by the national popular vote? It seems that such an amendment wouldn’t stand a chance. The Democratic states like, say, New York and California may favor it. The compact, though, makes a mockery of the idea that America is, in part, a confederation of sovereign states. So the big test of whether the Supreme Court is committed to the Electoral College is yet to come.