Analysis of 2016 election data found a large GOP advantage from intentional gerrymandering.

Texas state Sen. Juan “Chuy” Hinojosa looks at maps on display prior to a Senate Redistricting Committee hearing in Austin, Texas, in this May 30, 2013, file photo. CREDIT: Eric Gay/AP

Gerrymandering — the dark art of shaping legislative districts to give one party an electoral edge—gave Republicans an outsized advantage in races for the U.S. House and state legislatures in 2016, according to an analysis by the Associated Press published Sunday.

Republican candidates had other advantages, the AP found, from a larger number of incumbents to a voter base spread over more of the country rather than concentrated in cities. Even taking those into consideration, however, AP’s analysis found that gerrymandering handed the GOP a decisive advantage.

“The outcome was already cooked in, if you will, because of the way the districts were drawn,” John McGlennon, a professor of government and public policy at the College of William and Mary and a Democratic politician, told the Associated Press.

A report earlier this year by the Brennan Center for Justice at New York University came to similar conclusions. That analysis looked only at U.S. House races, while the AP analysis also includes state legislative elections.

“Partisan bias is distorting the composition of the U.S. House, and a handful of states are principally responsible for driving it,” the Brennan Center found. “The result in this decade’s maps has been a persistent and consequential seat advantage in favor of Republicans that will likely endure for the remainder of the decade.”

The AP report singled out Michigan, North Carolina, Pennsylvania, Wisconsin, Florida, and Virginia as examples of battleground states where Republican-led redistricting after the 2010 census led to a large GOP advantage in elections for the U.S. House and state legislatures.

Democrats held a partisan advantage in races in Maryland, Colorado, and Nevada, the analysis found, where legislative districts were re-drawn by a Democratic-majority legislature, a Democrat-led commission and by a court, respectively.

Still, experts say, Republicans generally benefit far more from gerrymandering than Democrats, since they hold control of the majority of state legislatures and governors’ mansions.

“There are significantly more pro-Republican maps at the moment than there are pro-Democratic maps,” Nick Stephanopoulos, a University of Chicago law professor who helped create the statistical method used in the new AP analysis, told the Associated Press.

Data suggests this partisan advantage is no accident. In Michigan and Wisconsin, for example, the partisan advantage gained by gerrymandering was unlikely to occur by chance, according to a separate analysis AP commissioned from the Princeton University Gerrymandering Project.

Gerrymandering has been in the headlines recently, with the issue even landing a coveted spot on political comedian John Oliver’s HBO show Last Week Tonight. The Supreme Court has agreed to hear a case, Gill v. Whitford, that could finally settle whether gerrymandering based purely on political party—rather than on race—is unconstitutional.

It usually works by either splitting up voters from an opposing party across several districts to dilute their vote—called “cracking”—or concentrating them in one district so their votes are over-concentrated on a single race — called “packing.”

Republican critics of Democratic concerns over gerrymandering point to what they say is Democrats’ poor campaigning, their concentration in cities—a sort of “voluntary gerrymandering,” some say—and their lack of incumbents to explain the Republican voting advantage.

Experts interviewed by the Associated Press acknowledged those factors, but the new analysis shows partisan gerrymandering also likely plays a significant role. That was especially true after the 2010 census, when new Republican majorities took control of the redistricting process.

“In 2011, the gerrymander was the most artful that I’ve seen,” Terry Madonna, director of the Center for Politics and Public Affairs at Franklin and Marshall College, told AP.

Report: Gerrymandering gave Republicans advantage in House, state elections was originally published in ThinkProgress on Medium, where people are continuing the conversation by highlighting and responding to this story.