The process of redrawing district lines to give an advantage to one party over another is called "gerrymandering." Here's how it works. (Daron Taylor/The Washington Post)

Last week the National Democratic Redistricting Committee, a group led by former U.S. attorney general Eric Holder Jr. with support from former president Barack Obama, announced its plan to put Democrats in governors' mansions and legislative chambers in 12 states in this year's elections. The group's goal is to break up Republican-held monopoly control of the nationwide redistricting process that will begin after the 2020 Census.

“In 2011, Republicans created gerrymandered districts that locked themselves into power and shut out voters from the electoral process,” Holder, the group's chairman, said in a statement. “By focusing on these state and local races, we can ensure Democrats who will fight for fairness have a seat at the table when new maps are drawn in 2021.”

But Holder's group is facing an uphill battle. Republicans are in complete control of the offices in charge of redistricting in 21 states, meaning they either A) control the governor's mansion and both chambers of the state legislature or B) they have veto-proof majorities in both legislative chambers regardless of the governor's party. Democrats maintain a similar advantage in just five states.

In the overwhelming majority of states, redistricting is handled just like any other piece of legislation. Lawmakers in legislative chambers work together to draw maps and submit them to the governor for approval. The party in control of the government also controls the redistricting process.

This creates a strong incentive for legislators to draw districts to their own advantage, particularly in states where one party controls all three branches of government or has veto-proof supermajorities in the legislative chambers. In many cases that's exactly what they've done.

In Pennsylvania, Republicans drew themselves a map giving the GOP control over 13 of 18 U.S. House seats, even though Republicans typically receive roughly 50 percent of the statewide popular vote. Republicans in North Carolina pulled off a similar feat with their redistricting, as did Democrats in Maryland.

Overall, Republicans had more monopoly control over redistricting nationwide in 2011 and, hence, were able to draw themselves a greater advantage in the U.S. House of Representatives. This disparity in redistricting power is particularly pronounced when expressed in congressional seats: Republicans maintain control over the redistricting process for 204 House seats, while Democrats control just 29. That ratio has grown less favorable to Democrats since the last redistricting when Republicans drew borders for 210 seats and Democrats drew 44.

These ratios will change between now and the 2021 redistricting — affected by the elections this year and in 2020. That's where Holder's group hopes to make its mark, saying in a recent news release that 2018 is “a critical election year for redistricting because it is the first cycle where the officials elected will serve during the redistricting process in 2021.”

To that end, they're focusing on races in states such as Ohio, Pennsylvania and North Carolina — places where GOP gerrymanders in 2011 led to extremely lopsided congressional maps that are still working through the courts.

While Holder's group is focused on breaking GOP legislative monopolies, other groups are working to reform the process by taking redistricting out of the hands of politicians entirely. In six states, accounting for 88 U.S. House seats, redistricting is handled by independent bipartisan commissions.

“We think the independent commissions in Arizona and California present models for reform from which other states can learn,” Patrick Rodenbush, a spokesman for the National Democratic Redistricting Committee, said in an email. “That being said, every state is different and a reform that is possible in one state may not work in another. We will continue to make determinations on reform proposals on a case-by-case basis.”

A survey released this week by Public Policy Polling found that 56 percent of Americans say they support changing the law so that congressional districts are drawn in a nonpartisan fashion. Just 12 percent said they were opposed; 32 were percent unsure.