A new report warns that district boundaries shaped to favor Republicans could make it tough for Democrats to retake the House in the November midterms.

While the political environment is expected to boost Democratic candidates, the party would need to beat Republicans by a margin of 11 points nationally, according to a report released Monday by the Brennan Center for Justice.

“Even a strong blue wave would crash against a wall of gerrymandered maps,” the report reads.

The 2010 Republican wave boosted the GOP’s control of statehouses across the country, enabling them to reshape district boundaries to make it easier for Republican candidates to win the seats. The report analyzed recent elections to measure “responsiveness,” or how the votes a political party receives correlates to how many seats it picks up.