To understand gerrymandering is to understand REDMAP, the concerted effort by the Republican State Leadership Committee of the United States to increase Republican control of Congress, as well as state legislators by controlling electoral boundaries. Special focus was given to Pennsylvania, Michigan, Wisconsin, Ohio and North Carolina – historically blue states that Republicans have swung red with appropriate redistricting. REDMAP launched in 2010 with previously unavailable mapping software, at an estimated cost of $30 million.
Source: Wikipedia
Additional Reading: This is how the GOP rigged Congress: The secret plan that handcuffed Obama’s presidency, but backfired in Donald Trump