The Pennsylvania Supreme Court has redrawn the state's congressional lines after declaring the current map an unconstitutional gerrymander.
The new lines are good news for Democrats. Under the old map, for example, Hillary Clinton won six of the state's 18 congressional districts. Under the new map, she would have won eight of the state's 18 congressional districts.
Still, the importance of the new map should not be oversold.
While the new map gives Democrats a better chance of taking back the House in 2018, it doesn't change the odds greatly.
You can do different types of fancy calculations, but Democrats are now probably favored in two seats that were at best toss-ups for them before: the new PA-5 and PA-6. Clinton would have won these districts by around 28 percentage points and 9 percentage points respectively under the new lines. Under the old lines, these were seats were carried by Clinton, but by just 2 percentage points or less.
A shift of two seats isn't small, though it's not exactly that large in the grand scheme. Democrats need a net gain of 24 seats to win the House in November. Two seats is less than 10% of that. You could certainly imagine instances where two seats made the difference in control of the House, though they are few and far between.
Now, the new map is beneficial to Democrats in ways beyond PA-5 and PA-6. The advantage that incumbents usually enjoy will be lessened by the new map because some Republican representatives will have many new and different constituents. Additionally, a few other seats shifted a few more points Democratic (e.g. the new PA-1), while the new PA-17 (represented currently by Keith Rothfus) shifted greatly toward the Democrats. It was won by Donald Trump by 3 points compared to 21 percentage points under the old lines.
Still, the political environment heading into 2018 will probably limit the effects of the new map. Because the national political environment is supposed to heavily favor the Democrats (for example, Democrats hold a 7-percentage-point edge in an average of recent live interview generic ballot polls), the incumbency advantage was already likely to be smaller. That's part of the reason why there were many Republican-held seats in Pennsylvania that Democrats already had a good chance of flipping under the old map.
Under the old map, the CNN House ratings had five Republican seats that were ranked lean Republican or worse for the Republicans. A sixth seat was considered likely Republican (i.e. a seat that could flip, but most likely not). Interestingly, that sixth seat, held by Rep. Lloyd Smucker, went from being won by Trump by 7 points to being won by Trump by 26 points. In other words, it's now much safer for the Republicans.
Let's take a closer look at the new map, taking into account the fact that Democrats hold about a 7-point lead on the generic congressional ballot. If you were to look only at the number of Republican seats that were won by Trump by 7 points or less or won by Clinton in 2016, the change from the old map to the new map is minimal. There were five Republican seats that met this criteria under the new map and four under the old map. Democrats will certainly welcome the one additional seat, but one seat is clearly not a game-changer.
Nationally, the number of Republican seats that are in danger of being flipped barely expanded under the new map. For instance, the number of seats won by Trump by 7 points or less or won by Clinton went from 45 to 46. The more limited playing field of Republican-held seats won by Clinton went up from 23 to 25.
It's also still the case that Democrats will have to win the national House vote by a fairly wide margin to take back the House.
Previously, I've calculated that to probably be between 5.5 and 8 percentage points. Based on some simplistic math that takes into account how many seats are now vulnerable for the Republicans and the fact that the party that has benefited from a wave has netted a pickup of approximately two-thirds of the number of vulnerable seats in the past few wave elections, Democrats may now only need to win the national popular vote by between 4.5 and 7 percentage points to take back the House. Again, that's better than it previously was for the Democrats, but it's a considerable margin and not all that different from what they needed before the Pennsylvania map was redrawn.
If Democrats want to greatly increase their chance of taking back the House, what they'll want is a change in the national political environment. Their generic ballot lead has fallen from a double-digit lead in December. If Democrats can get that lead back up to this margin, it would have a far greater impact on their hopes of taking back the House than the new map out of Pennsylvania will.