The Pennsylvania Supreme Court ruled last month that the state’s US House of Representatives map was based on a Republican partisan gerrymander that violated the state’s constitution — and struck the map down. But the state GOP made a last ditch appeal to the US Supreme Court, hoping justices would stay the decision.

That effort has now failed. On Monday, the US Supreme Court announced that it wouldn’t put the Pennsylvania ruling on hold — a decision that effectively makes clear the state will get a new House of Representatives map for this fall’s elections.

Pennsylvania’s government now has until February 15 to get a new map through the legislature and signed into law. If they fail to do so — a likely prospect, since the state has a Republican-controlled legislature and Democratic governor — the Pennsylvania Supreme Court will take over the process and institute a new map. (The court has a Democratic majority.)

The new Pennsylvania map has the potential to be a very big help to Democrats’ efforts to regain control of the House of Representatives in 2018, because the state’s old map was one of the most biased toward Republicans in the country.

To get a sense of how powerful Pennsylvania’s gerrymander was, consider that, in 2012, Democratic candidates won slightly more votes in US House elections and Barack Obama won the state. But the state’s 18 House seats didn’t split 9-9 between the parties — instead, Republicans won 13 seats there, and continued to win them for the rest of the decade.

Republicans currently hold the majority in the entire House of Representatives by 24 seats (assuming no special elections result in partisan change) — and the Pennsylvania gerrymander could be responsible for three to four of those seats. That’s a massive amount when you keep in mind that it’s just one state.

Yet just how much the new map will improve the situation for Democrats depends on its specifics. After all, simple geography (where Democrats and Republicans have chosen to live) makes a 9-9 map difficult to put together based on existing political borders.

Still, the ruling comes at a time when Democrats already sensed opportunity in Pennsylvania. A combination of retirements, scandals, and suburban voters’ repulsion of Trump have rattled the GOP’s House delegation there. The existing map was so tough for Democrats that they were wary of setting their sights too high — so a new one could be a major help.

Pennsylvania’s House map was a contender for “gerrymander of the decade”

Republicans dominated in the 2010 elections in Pennsylvania, winning control of the governorship and the state house, and holding on to the state senate. So when the once-a-decade redistricting process kicked off the following year, the GOP was in a powerful position. The party could redraw the state’s US House of Representatives districts however it liked, cutting Democrats out of the process entirely.

You’ll notice that the map above isn’t particularly clean. It’s full of jagged edges, weird outcroppings, and strange shapes. That’s no accident: Republicans tried to pack Democratic-leaning areas together into very few districts. The hoped-for result was that the GOP would lose a few districts by large margins, yet win a majority of districts comfortably and consistently.

That’s exactly what happened. In statewide elections, Pennsylvania was a competitive swing state. But in all three US House election years since, the partisan split of the results has been completely unchanged: 13 Republicans have won, and just 5 Democrats have. (This was a particularly stunning result in 2012, when Barack Obama won statewide, and Democratic candidates won more votes in House elections than Republicans did.)

As far back as 2011, Sean Trende of RealClearPolitics suggested Pennsylvania’s map could be “the gerrymander of the decade.” And a 2017 report by the Brennan Center concluded that Pennsylvania, Michigan, and North Carolina’s House maps had “the most extreme levels of partisan bias” in the country, and estimated that Pennsylvania delivered Republicans three or four extra seats on average.

Still, some electoral analysts — including Trende — don’t think it’s likely that Pennsylvania will now adopt a map that entirely rolls back that advantage. For instance, if the new maps are drawn to try to make districts extremely compact while trying to avoid splitting existing political borders (like town lines), Democrats might not even be helped all that much. So we don’t yet know just how big an impact this will have.

The Pennsylvania GOP’s congressional delegation is already in turmoil

But all this is unfolding during a difficult time for Pennsylvania House Republicans.

Rep. Tim Murphy (R) of the 18th district resigned when news broke that the pro-life lawmaker had asked a woman he was having an affair with to have an abortion. The special election there is set for March 13 (and will proceed as scheduled despite the court ruling). It’s a strongly Republican district, but the GOP fears they could lose the seat to the Democratic nominee, Conor Lamb, a Marine veteran and former prosecutor.

Rep. Pat Meehan (R) of the 7th district, is retiring after the New York Times reported he used taxpayer dollars to settled a misconduct complaint from a former aide who complained he made unwanted overtures toward her.

Rep. Charlie Dent (R) of the 15th district, a frequent critic of Donald Trump and co-chair of the Tuesday Group, is also retiring.

The coming new map will scramble all this. Members of Congress representing a district, or challengers planning to run against them, may find out they’re suddenly in an entirely different district. Some incumbent members of Congress could end up being put in the same district.

Considering just how important the Pennsylvania gerrymander has been for Republicans’ control of the House — and considering that a Democratic governor and then a Democrat-controlled Supreme Court will have veto power of the new map — it seems all but certain that the new map will help Democrats. But we’ll have to actually see the new map to get a better idea.