ARI SHAPIRO, HOST:

Pennsylvania's congressional delegation could be completely upended this fall. The state just got a new district map. The old lines are a classic example of partisan gerrymandering. Pennsylvania voters are almost evenly divided between the two parties, but Republicans consistently win 13 of the state's 18 congressional seats. To discuss this new map, Dave Wasserman of The Cook Political Report joins us now. Hi there.

DAVID WASSERMAN: Hey. Thanks for having me.

SHAPIRO: Around the U.S., there are several fights over congressional maps right now. Explain how the fight in Pennsylvania started.

WASSERMAN: Sure. Well, Democrats have been frustrated all decade with Pennsylvania's map. Now, Democrats are at a disadvantage to begin with because most of their voters in Pennsylvania live in Philadelphia and Pittsburgh, and so it's difficult for them to win a broad swath of territory, a broad swath of districts. But Republicans compounded that problem because they packed Democrats even further into just a few districts.

But Democrats really won a victory in state Supreme Court races in 2015 that allowed them to have a favorable majority on the state Supreme Court. And even though the Supreme Court of the U.S. has had a hard time defining what constitutes partisan gerrymandering, Democrats were able to sue in Pennsylvania to overturn the map on the grounds that it violated the state constitution. And the U.S. Supreme Court refused to step in and intervene on Republicans' behalf in Pennsylvania. So it looks like this map is going to be in place for 2018.

SHAPIRO: So since this is the radio, describe what the old congressional map looked like visually compared to the new one.

WASSERMAN: The map that Republicans put in place in 2012 looked like a work of abstract art.

SHAPIRO: (Laughter).

WASSERMAN: There was a district outside of Philadelphia that looked like Bullwinkle the moose. There were districts that stretched all the way from the northeastern corner of the state to central and western Pennsylvania. The new map forms lines that are much more regular. And even though there are still some odd shapes, all of the districts are pretty legible to the naked eye as opposed to the old boundaries which could be really hard to discern.

Democrats were poised to pick up seats already in 2018 even with the Republican gerrymander in place. But this really strengthens their pickup opportunities and could probably add on one or two seats to what they were expected to gain under the lines that were already in place.

SHAPIRO: What does that mean for the national Democratic Party's chances of taking back the House of Representatives in November?

WASSERMAN: Let's put it this way. Democrats need to pick up 24 Republican seats to win back the House. And in Pennsylvania, under this new map, they could have a shot at picking up up to six seats. That's a quarter of the seats they need nationally. So this is a huge deal for Democrats.

SHAPIRO: Pennsylvania Republicans are promising to challenge this. Do you think they have any hope of blocking it?

WASSERMAN: Well, look. The state Supreme Court in adopting this remedial plan - they didn't just undo Republicans' gerrymander. They went further by making decisions that helped Democrats compensate for their natural geographic disadvantage in the state, and that's a bit different from what you might expect from a purely partisan blind map drawing process. So Republicans do have reason to be upset. They see this as unfair. Now, the U.S. Supreme Court has already declined once to intervene on Republicans' behalf to stop this remapping process, so it's unlikely they will now that the map is in place.

SHAPIRO: David Wasserman of The Cook Political Report, thanks for joining us today.

WASSERMAN: Thanks so much. Transcript provided by NPR, Copyright NPR.