(CNN) On Monday, the Supreme Court rejected a Republican effort to halt a required redrawing of the state's 18 congressional districts before the year's midterm elections.
That is a very big deal. Here's why.
Republicans have controlled the line-drawing process in Pennsylvania for the last two decades. That control has created a congressional map that heavily favors Republicans -- they control 13 seats to the Democrats' five -- despite the fact that the state is a Democratic leaning one. (Before Trump carried Pennsylvania in 2016, George H.W. Bush in 1988 was the last GOP presidential candidate to win the state.)
Earlier this year, the Pennsylvania Supreme Court said the map was drawn primarily for political purposes -- and, therefore, violated the state's constitution. The state Supreme Court ordered the state legislature, which is still controlled by Republicans, to make a new map for the 2018 election -- and have it approved by Democratic Gov. Tom Wolf.
Republicans had hoped to avoid that fate with their appeal. That hope ended at the Supreme Court today.
"Pennsylvania was already hosting a handful of competitive races, but a brand new congressional map is the ultimate wild card," said Nathan Gonzales, who runs the non-partisan "Inside Elections" tip sheet. "Not only could it weaken Republicans' incumbency advantage in key districts, but it could scramble the fields of Democratic challengers trying to replace them."
Gonzalez's "Inside Elections" already rates six Republican-held seats as competitive heading into the 2018 midterms; one seat -- Pennsylvania's 7th, which embattled Rep. Pat Meehan (R) is abandoning, is rated as "tilting" toward Democrats while 5 GOP seats are in the "lean Republican" category.
CNN's Key Race ratings for the House similarly list one Pennsylvania Republican seat, Meehan's, as "Tossup" and four Republican seats as "Lean Republican."
While Gonzalez is right that we can't know what the map will look like -- and who it will hurt most -- it's virtually impossible to see how Republicans can hold 13 seats after the election in Pennsylvania given how the current map was drawn.
"The plan ruthlessly sewed the state, particularly the Philadelphia suburbs, into a crazy quilt," reads the Almanac of American Politics on the map. "Montgomery County, about the population of one district, was split five ways to boost three suburban Republicans, who were happy to feed their trickiest inner suburbs to Philadelphia's Democrats."
A look at the 2016 presidential results bears that point out. The 6th, 7th and 8th districts -- all of which encompass the Philadelphia suburbs -- are held by Republicans but were won by Hillary Clinton in the election. Even slight tweaks to those seats would make it close to impossible for one -- or more than one -- of these seats to be held by Republicans.
There are also two other districts in northeastern Pennsylvania -- the 15th centered on Allentown and the 16th that has Lancaster as its base -- in which Trump won with 51% and 50%, respectively, in 2016. The 15th district is open with Republican Rep. Charlie Dent retiring, while the 16th district is represented by Rep. Lloyd Smucker, who won with 54% in 2016. Moving the lines of their districts around could endanger one or both seats.
That's a whole lot of potential vulnerability for Republicans that, with the necessity of creating a new map now certain, is unavoidable for GOP line-drawers.
If there are six -- or even more -- potential Democratic gains in Pennsylvania, that could get the party one quarter of the way back to the majority. Which is a very big deal.