WASHINGTON (AP) — Republicans helped solidify their hold on the U.S. House when GOP legislators in key states drew new congressional maps after the 2010 Census. But was it geography or gerrymandering that gave Republicans an edge in state after state?
The Republican advantage is significant. In the 2012 election, Democratic candidates for the House received 1.4 million more votes than their Republican opponents, yet the GOP maintained a 33-seat majority. It was only the second time since World War II that the party receiving the most votes didn't win a majority of House seats.
It is an edge the Republicans will carry into November's midterm elections and perhaps beyond.
Democrats blame Republicans for drawing districts that favor GOP candidates. "We've never seen a level of gerrymandering as severe as in 2010," said Matt Bennett, a former aide to President Bill Clinton.
But some experts say geography may be just as responsible as gerrymandering for the Republican advantage.
Living patterns in many states make it relatively easy to pack Democrats into fewer congressional districts because they are much more likely than Republicans to live in dense urban areas, said Norman Ornstein, an expert on Congress at the American Enterprise Institute. Republicans are more likely to live in suburban areas, which can be more politically diverse, or in geographically large rural areas.
"Democrats are at a disadvantage here," Ornstein said. "It's not as difficult for Republicans doing a redistricting to concentrate the Democrats in districts where they can win 80 or 85 percent of the vote."
Gerrymandering is the practice of drawing election districts in a way that gives an advantage to one political party over another. One goal is to pack voters from the other party into the fewest districts possible. It often leads to odd-shaped districts that split counties, cities and even neighborhoods.
If done well, gerrymandering can protect incumbents and maximize the number of districts in which one party has a majority of voters.
But aggressive gerrymandering isn't always necessary for Republicans to gain an edge, said Thomas Mann, an expert on Congress at The Brookings Institution.
"A lot of the Republican advantage — by no means all of it — a lot of it is the result of living patterns," Mann said.
Two political scientists, Jowei Chen at the University of Michigan and Jonathan Rodden at Stanford University, are popularizing the idea that geography is more important than gerrymandering. They did a study using computer simulations to see whether the political makeup of Congress would be any different if congressional maps were drawn without partisan bias.
"In the vast majority of states, our nonpartisan simulations produced Republican seat shares that were not much different from the actual numbers in the last election," Chen and Rodden wrote in a New York Times article. "In short, the Democrats' geography problem is bigger than their gerrymandering problem."
Not so fast, said Michael McDonald, a political scientist at George Mason University. Living patterns only matter if districts are required to be geographically compact. And even then, McDonald said, it is not hard to draw maps that favor Democrats.
Congressional districts are large, averaging more than 700,000 people. That makes it easy in most cities to combine urban, suburban and rural voters within a compact district, McDonald said.
"If Democrats controlled the process, they could draw pretty much whatever they wanted to draw," McDonald said. "It's just who controls the pen."