Republicans managed to maintain their majority in the U.S. House after the 2012 election—in which they lost the presidential contest by five million votes—thanks to considerable gerrymandering of competitive congressional districts that were padded with GOP voters. But this redistricting in many key battleground states did not make seats invincible to a Democratic challenge, especially if turnout among GOP-friendly voters is weak in the 2014 election.

No one knows how the GOP will be viewed this time next year. But right now the Republican brand is hurting following the decision to shut down the government, and risk a default on the U.S. debt, over funding for the Affordable Care Act (pdf) (aka Obamacare). Numerous polls reveal a large majority of Americans disapprove of the party, and if that lack of support lasts for the next 12 months, the GOP could lose up to 10 seats in states where gerrymandering played a key role in the 2012 campaign.

Eric Chemi, head of research for Businessweek and Bloomberg TV, says there are 11 states where the Republicans are most vulnerable to losing seats next year because redistricting spread the GOP base a little thin.

Pennsylvania tops the list with three districts at risk, followed by Ohio, North Carolina, Michigan, Virginia, New Jersey and Indiana with between one and two seats each possibly in danger of flipping, if turnout does not favor Republicans. In Pennsylvania in 2012, Republican congressional candidates won 49% of the vote, but gained 72% of the seats. Throwing in Ohio, North Carolina, Missouri and Virginia, the Republican and Democrats split the combined vote almost evenly. Yet in these five states Republicans won 51 seats to only 21 for Democrats.

“The effect of gerrymandering has allowed for extremely solid Democratic districts, while spreading the Republican support thin,” Chemi wrote. A decline of only 5% to 10% in GOP voter support “could easily result” in 10 lost seats, he says.

Ten seats might not seem like a lot. But it is when Republicans control only 31 more districts than Democrats (231 v. 200).

-Noel Brinkerhoff

To Learn More:

Ranking the GOP's Most Vulnerable States in 2014 (by Eric Chemi, Bloomberg Businessweek)

Republicans Win Congress as Democrats Get Most Votes (by Greg Giroux, Bloomberg)

The Great Gerrymander of 2012 (by Sam Wang, New York Times)

In Pennsylvania, the Gerrymander of the Decade? (by Sean Trende, Real Clear Politics)