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"It is fair to say the input received from incumbent members of Congress urged caution," said Erik Arneson, spokesman for Senate Majority Leader Dominic Pileggi, R-Delaware. "With the exception of the Rothfus seat, the sense was there was not any Democratic incumbent member of Congress particularly vulnerable."

There are 1.1 million more Democrats in the state than Republicans, and even with the new maps, Democrats have a registration edge in 12 of the 18 seats. But the advantages for the GOP come in voter performance instead of registration: In the 12th District, for instance, 54 percent of voters supported John McCain in 2008, despite it being only 37.5 percent Republican. On Nov. 6, the district would give Mitt Romney a 17 percentage-point edge over Mr. Obama (winning it 58.5 to 41.5 percent), helping Republican challenger Mr. Rothfus beat Mr. Critz by 3 points (51.5 to 48.5 percent).

In the last round of redistricting after the 2000 census, GOP mapmakers, in an attempt to pick up additional seats, made Pennsylvania's districts too close. The bid worked in the first try in 2002 with their party taking 12 of 19 seats. By 2006, things changed with wins by candidates such as Mr. Altmire, and Democrats took 11 of the seats.

In 2000, Republicans "overreached. They were so greedy and did it in such a way that when the worm turned, they couldn't stop it," said U.S. Rep. Mike Doyle, D-Forest Hills. "They learned from that, unfortunately, and didn't make that mistake this time."

That is part of the reason why Mr. Doyle and some other Democratic leaders, including Mr. Altmire, urged approval of the new GOP-designed maps in 2011. Under the circumstances, they felt they got the best deal they could: three solidly Democratic districts in and around Philadelphia; one taking in all of Pittsburgh represented by Mr. Doyle; and another bolstering the Democratic vote in the District 17 seat formerly held by Tim Holden, in exchange for boosting the GOP vote in the District 11 seat that U.S. Rep. Lou Barletta, R-Luzerne, had taken from longtime Democratic incumbent Paul Kanjorski.

"Barring us drawing it, there was no way to make it better than it was. None of us liked the map, but it wasn't going to get better for anybody," Mr. Doyle said. "Everybody knew it was about the best that was going to happen."

It wasn't always this way. From the end of World War II through 1994, Democrats nationally won a bigger percentage of House seats than their popular-vote total, probably due to the prevalence of old-time Southern Democrats. That flipped in the Republican Revolution year, and the GOP has outperformed its popular-vote total in the two decades since. The GOP's nearly 6 percent gap between votes and seats won on Nov. 6 was the biggest for either party since the Democrats last did it in 1992, according to data from the University of Virginia Center for Politics.

Even if Democrats had a say in drawing Pennsylvania's districts, it might not have mattered, given the difficulty of chopping up cities where their voters live. It's a lot easier for Republican mapmakers to do the exact opposite -- that is, mass Democratic voters together in urban areas and then compete for those left in suburbs and rural areas.

Look at the Pennsylvania results Nov. 6: The five Democratic congressional winners took an average of 76 percent of their district vote, while Republicans won their 13 seats with an average of 54 percent.

"The key way a gerrymandering party is able to win more than its share of seats is to pack the opposite partisans into a district where they control a huge amount of the vote and essentially waste their vote. It's almost impossible to draw a district where Republicans win 80 percent of the vote," said Mr. Goedert of Washington University. With a few exceptions in states such as Texas, he said, "there aren't areas of Republican voting strength that are so overwhelmingly Republican that they vote 90 percent Republican, as opposed to urban districts that vote 90 percent Democratic."

Mr. Doyle argued that all is not lost for the Democratic delegation going forward. They could keep trying to flip the three GOP-held seats around Philadelphia (Districts 6, 7 and 8) where voter performance is near even and try to win back the two Western Pennsylvania seats (Districts 3 and 12) that have gone Republican since 2010.

"These are seats we can win with the right candidates," Mr. Doyle said.

With the maps set in place through 2022, that is one thing he and the other party can agree on.

"Obviously, wherever you put the line makes a difference, but people said the same kind of things 10 years ago," said Mr. Arneson, the state Senate staffer. "In the end, voters choose between two candidates, and who those candidates are matters."