“Using data from the 2012, 2014, and 2016 election cycles, Extreme Maps finds that partisan bias resulting largely from the worst gerrymandering abuses in just a few battleground states provides Republicans a durable advantage of 16-17 seats in the current Congress, representing a significant portion of the 24 seats Democrats would need to gain control of the House in 2020. These “extreme maps” were all drawn in states under single-party control; the report finds that conversely, maps drawn by independent commissions, courts, or split-party state governments had significantly less partisan bias in their maps.”

Read the full report: https://www.brennancenter.org/sites/default/files/publications/Extreme%20Maps%205.16_0.pdf