The moose-shaped 7th Congressional District of Pennsylvania is a pretty egregious example of a partisan gerrymander. Illustration: OpenStreetsMap via Fair Districts PA

All eyes have been on the U.S. Supreme Court to decide at some point this year whether state redistricting decisions could be so partisan as to violate voters’ constitutional rights, absent some showing of racial discrimination. In the past, purely partisan gerrymandering was considered kosher as a political decision that courts had no basis to challenge. That’s now under question, in part thanks to new methods for measuring the impact of redistricting on voting rights. It was generally deemed unlikely that SCOTUS would decide cases from Wisconsin and Maryland in time to affect maps for this year’s midterm elections.

But now, in a state whose Republican legislators have become legendary gerrymanderers, a state court acting on its state Constitution has suddenly overturned a congressional map and is demanding immediate compliance, upsetting GOP midterm plans:

The Pennsylvania Supreme Court on Monday ruled that the state’s congressional map went so far to benefit Republicans that it violated the state constitution.

The court, where Democrats have a 5-2 majority, blocked the use of the map in the 2018 midterm elections and ordered state lawmakers to begin to draw a new map.

The Pennsylvania congressional map has been notorious since its first use, in 2012, when Republicans won (and have subsequently held) 13 of the state’s 18 House seats despite losing a majority of the popular vote. Republican legislative leaders made no bones about their skill in screwing over Democrats. That didn’t much matter until now, when the Pennsylvania Supremes have ruled that extreme partisan gerrymandering violated both equal protection and “free expression” provisions of the state Constitution.

Because the decision was made on the basis of the state, not the U.S., Constitution, SCOTUS is very unlikely to intervene to save the bacon of Pennsylvania Republicans. If there is any federal recourse against the decision, it could involve the remedy the Pennsylvania court imposed. The judges gave state lawmakers only until February 9 to draw up a new map, with the governor having only until February 15 to submit it to the court, which will draw its own map if the state fails to make its deadline. The speedy timetable is intended to make it possible for the state to hold congressional primaries in May as is currently scheduled. It also reflects the fact that with today’s redistricting software, it really doesn’t take that long to redraw maps. But that’s still a tight window, and with a Republican legislature and a Democratic governor, an agreement could be difficult to reach.

Any map that complies with the ruling will likely cost the GOP one or more House seats at a time when they are already in danger of losing control of the chamber:

Less partisan lines could give Democrats a chance to win back as many as half a dozen seats that had been lost to them over the past decade. It could also give the party a major boost in its quest to take back the entire House of Representatives in November.

As ThinkProgress notes, there is one aspect of the ruling that could mitigate GOP losses:

One bright light for Republicans in Monday’s order is that it requires districts to be “composed of compact and contiguous territory” and that they “do not divide any county, city, incorporated town, borough, township, or ward, except where necessary to ensure equality of population.” Because Democrats tend to live clustered together in cities, while Republicans tend to be more spread out across suburbs and rural areas, a requirement to draw compact maps that incorporate county and municipal boundaries will tend to favor Republicans.

Still, even under the current map, Republican had four Pennsylvania seats considered vulnerable. That picture is almost certain to get worse.