It’s really been hard to get a fix on this, since everyone’s quite naturally focused on the more immediate explosions that could enamate from the U.S. Supreme Court later this month. But SCOTUS’ surprising agreement to hear Evenwell v. Abbot, a challenge to the traditional understanding of “one person, one vote,” could be a pretty big deal if the Court agrees with the plaintiff’s suggestion that legislative districts at the federal and state level should be balanced according to the number of voting-age citizens rather than total population (which is the universal practice now).

At FiveThirtyEight last week, Harry Enten and David Wasserman took an initial look at the potential consequences of this kind of shift:

A move toward counting only eligible voters, as logistically difficult as it may be, would drastically shift political power away from the urban environs with minorities and noncitizens, and toward whiter areas with larger native-born populations. That’s bad news for Democrats: Of the 50 congressional districts with the lowest shares of eligible voters, 41 are occupied by Democrats (nearly all are Latino-majority seats). Meanwhile, of the 50 districts with the highest shares of eligible voters, 38 are represented by the GOP.

And that’s only looking at Congress. A new standard could similarly affect state legislative districts, too.

Today Sean Trende of RealClearPolitics looks at the issue and mentions an entirely different effect that a surprise decision to upend “one person one vote” could have:

If the court were to find for the plaintiffs – and it seems unlikely that the court would have gratuitously taken up this case, absent a circuit split, if there weren’t some substantial support for the plaintiffs’ position – it would mean that, once again, virtually every legislative and congressional district in the country would have to be redrawn (although this would not, as some have suggested, affect apportionment – i.e. the number of seats allocated to each state). This would occur at a time when Republicans control a record-high number of state legislatures and a majority of state governments. Republicans would be able to update their maps to account for changes in political orientations in their states since the previous round of redistricting.

At present the next redistricting is due to occur after the 2020 elections, a presidential year when, all other things being equal, Democrats should do better than in a midterm. Throwing in an extra redistricting before then with a new, Republican-friendly standard would certainly help retrench the vast GOP gains of 2010 and 2014, though the exact timing would determine the exact impact. Since SCOTUS just took the case, it won’t be decided until 2016, and presumably a decision could not be implemented overnight. But in any event, the litigation could be a hidden time bomb in American politics, and a possible calamity for progressives at a time when they already face plenty of challenges.

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Ed Kilgore is a political columnist for New York and managing editor at the Democratic Strategist website. He was a contributing writer at the Washington Monthly from January 2012 until November 2015, and was the principal contributor to the Political Animal blog.