PHOTO: Last updated: March 15, 2018 | CNN Politics
Now playing
01:54
Key House races to watch in 2018
senator bill nelson 11132018
senator bill nelson 11132018
PHOTO: CNN
Now playing
02:03
Nelson: Scott should recuse himself
Rep. Marsha Blackburn, R-Tenn., speaks to supporters after she was declared the winner over former Gov. Phil Bredesen in their race for the U.S. Senate Tuesday, Nov. 6, 2018, in Franklin, Tenn. (AP Photo/Mark Humphrey)
Rep. Marsha Blackburn, R-Tenn., speaks to supporters after she was declared the winner over former Gov. Phil Bredesen in their race for the U.S. Senate Tuesday, Nov. 6, 2018, in Franklin, Tenn. (AP Photo/Mark Humphrey)
PHOTO: Mark Humphrey/AP
Now playing
01:26
Meet the Republicans who held onto the Senate (2018)
Democratic congressional candidate Ilhan Omar is greeted by her husband
Democratic congressional candidate Ilhan Omar is greeted by her husband's mother after appearing at her midterm election night party in Minneapolis, Minnesota, U.S. November 6, 2018. REUTERS/Eric Miller
PHOTO: Eric Miller/Reuters
Now playing
01:33
Minorities, LGBT make history in 2018 midterms
PHOTO: WJLA
Now playing
02:01
Hear from the Democrats who took back the House
PHOTO: CNN
Now playing
01:50
Abrams vows to remain in gubernatorial race
PHOTO: Spectrum News NY1
Now playing
02:05
Ocasio-Cortez: This is a movement for justice
PHOTO: KXAN
Now playing
02:07
O'Rourke congratulates Cruz on his victory
PHOTO: CNN
Now playing
01:57
Tapper: This is not a blue wave
PHOTO: CNN
Now playing
02:11
Van Jones: It's a rainbow wave
LAS VEGAS, NV - OCTOBER 20:  U.S. Rep. and U.S. Senate candidate Jacky Rosen (D-NV) speaks during a rally at the Culinary Workers Union Hall Local 226 featuring former U.S. Vice President Joe Biden on October 20, 2018 in Las Vegas, Nevada. Early voting for the midterm elections in Nevada begins today. (Photo by Ethan Miller/Getty Images)
LAS VEGAS, NV - OCTOBER 20: U.S. Rep. and U.S. Senate candidate Jacky Rosen (D-NV) speaks during a rally at the Culinary Workers Union Hall Local 226 featuring former U.S. Vice President Joe Biden on October 20, 2018 in Las Vegas, Nevada. Early voting for the midterm elections in Nevada begins today. (Photo by Ethan Miller/Getty Images)
PHOTO: Ethan Miller/Getty Images
Now playing
01:45
CNN projects Jacky Rosen elected to US Senate
Additional Embargo:   Additional Source(s):    Date Shot: 11/6/2018   Shipping/Billing Info:     Description: Projects: None  Cost Center: Atlanta National Desk / 20100101   Created By: DHackett  On: 1541544461  --------------------------------------------------------------------------------
Additional Embargo: Additional Source(s): Date Shot: 11/6/2018 Shipping/Billing Info: Description: Projects: None Cost Center: Atlanta National Desk / 20100101 Created By: DHackett On: 1541544461 --------------------------------------------------------------------------------
PHOTO: WSB;
Now playing
00:59
Brian Kemp: Confident victory is near
2018 Elections Primary Clean path
2018 Elections Primary Clean path
PHOTO: CNN
Now playing
01:49
Pelosi: Tomorrow will be a new day in America
PHOTO: CNN
Now playing
01:36
The Democrats who might lead the House
PHOTO: CNN
Now playing
01:54
DeSantis thanks Trump in victory speech
PHOTO: KXAN
Now playing
01:44
Cruz: This election was a battle of ideas
(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.