Pennsylvania Democrats could see their prospects for taking control of the state House improve dramatically if newly proposed maps for redrawing legislative districts are finalized.

But in the state Senate, the new map would likely help entrench the Republican majority by largely protecting incumbents, effectively creating districts that favor the GOP more than the current map does, according to a detailed data analysis conducted for The Inquirer by the nonpartisan Princeton Gerrymandering Project.

Officials proposed the new maps Thursday, and a five-person commission approved them, kicking off a redistricting process that will shape power for the next decade and is already sending political sparks flying. The Senate map passed unanimously, 5-0. The House map advanced 3-2, with Republicans opposing it and the commission chair appointed by the Democratic-controlled state Supreme Court joining the two Democrats in supporting it.

That starts what could be a three-month-long process until final approval, beginning with a 30-day window during which people can file complaints and lawmakers can make changes.

Mark Nordenberg, who chairs the state’s Legislative Reapportionment Commission that drew the maps, stressed the difficulty of finding a compromise due to Pennsylvania’s inherent political geography, in which the southeastern corner of the state — mostly Philadelphia and its suburbs — accounts for the overwhelming majority of Democratic voters.

“It becomes almost impossible to spread them out to have a broader geographic impact,” Nordenberg said in his opening remarks before the vote. “My own experience is that there’s nothing easy about drawing these maps.”

The 203-member House, like the 50-member Senate, is currently controlled by Republicans. Whether that changes in next year’s midterm elections or in the years ahead will depend partly on the new maps. The party that controls the legislature, which Republicans have mostly dominated for decades, has the power to make policy affecting millions of Pennsylvanians.

Democratic Gov. Tom Wolf has wielded his veto pen to block Republican legislation for years, but with the prospect that a Republican could succeed him after next year, even more focus will be on who controls the legislature.

» READ MORE: Is the new Pa. House map better for Democrats or Republicans? We tested it.

While the Senate map had bipartisan support, Republicans were quick to call the House proposal an “extreme partisan gerrymander” that could cost them control of the chamber. The current House map favors Republicans, and Democrats have blamed it for locking them out of power for a decade. The new one would also favor the GOP — but to a much lesser extent, according to the analysis.

J.J. Abbott, a Democratic strategist, credited the new House map with “undoing the gerrymandering of the past.”

“Clearly this was created in a way that wasn’t about prioritizing protecting incumbents and instead prioritizing competitiveness, which is the goal,” he said.

Republican lawmakers called the new House map an unfair over-correction. State House Majority Leader Kerry Benninghoff (R., Centre) said it would hand Democrats control of the chamber.

“The map before us is nothing short of a danger to our system of government that upends established norms and the emphasis on local control and local voices that Pennsylvanians hold dear,” Benninghoff said.

In many ways, Pennsylvania’s political geography inherently favors Republicans, with Democratic voters concentrated in a smaller number of districts. But the new House map is actually friendlier to Democrats than the state’s natural political geography is, according to the analysis.

To better understand these maps and others, The Inquirer is partnering with the Princeton Gerrymandering Project to analyze the districts. Using the two-party vote share average of the 2016 and 2018 presidential elections, and also informed by the 2016 and 2018 U.S. Senate results, the analysis scores and categorizes the underlying partisanship of each of the proposed new districts.

While there are currently 118 Republican and 85 Democratic districts in the base partisanship of the current House map, that would shift to 104 Republican and 99 Democratic districts under the new map.

House Republicans said the new map would draw 12 GOP incumbents into districts where they would have to face fellow lawmakers in primaries. It would only pit two Democrats against each other. Four Democratic incumbents could face four Republican incumbents in combined districts.

Unlike in U.S. House races, state legislative candidates are required to live in the district they’re seeking to represent, meaning incumbents drawn together have no choice but to challenge each other in a primary, move to a new district, or drop out.

» READ MORE: What to watch as Pennsylvania loses a congressional seat: ‘The stakes are really high’

The Legislative Reapportionment Commission is made up of the four Democratic and Republican caucus leaders of the House and Senate, and Nordenberg, the chair appointed by the Supreme Court. The other members are Senate Majority Leader Kim Ward (R., Westmoreland), Senate Minority Leader Jay Costa (D., Allegheny), Benninghoff, and House Minority Leader Joanna McClinton (D., Philadelphia).

Benninghoff pointed to several cities that are split — Harrisburg, Lancaster, State College, Reading, Allentown, and Scranton — as evidence of Democrats attempting to spread Democratic voters across more districts to win more seats.

“The only conclusion we can draw is they are split for purely partisan gain,” he said.

McClinton said the map fairly accounts for dramatic demographic and population changes since the last Census by creating three new House districts in Philadelphia, Lancaster, and Montgomery Counties.

“It also comports with the Voting Rights Act, which requires that communities of color must have the same opportunity as other members of the electorate to participate in the political process and to elect representatives of their choice,” she said.

Nordenberg said the new House map would create a path for voters to elect a more diverse legislature, with several districts redrawn to encompass populations where more than 50% of residents are Black, Latino, or another non-white group.

» READ MORE: Is the new Pa. Senate map better for Democrats or Republicans? We tested it.

State Sen. Vincent Hughes (D., Philadelphia), who heads the campaign arm for Senate Democrats, applauded the creation of a Latino-heavy district in the Lehigh Valley on the Senate map. The 50-member state Senate has no Latinos.

“I think this is going in the right direction,” Hughes said of the Senate map, calling it “much more fair.”

Christopher Nicholas, a GOP strategist, criticized the House map and questioned the bipartisanship of the commission’s chair. “Hard to see how [Mark Nordenberg] didn’t have his thumb on the scale for Democrats,” Nicholas said in an email.

Abbott, the Democratic strategist, dismissed complaints about splitting up cities and counties.

“Pennsylvania is pretty much a split state,” he said, “so the legislature’s makeup should be reflective of the state and it seems like this does a lot in terms of trying to get toward more fair and equitable representation.”

The state Constitution provides a 30-day window for the commission to make changes or for “any person aggrieved by the preliminary plan… to file exceptions with the commission.” The commission then has 30 days to respond to those challenges and make changes. That kicks off one more 30-day period for people to appeal the map directly to the Supreme Court.

That’s a long and potentially problematic runway, considering Pennsylvania has a primary election scheduled for May 17, and the deadline for filing paperwork to become a candidate is in February. The redistricting process started late because of delays in releasing the Census data needed to draw the districts.