Here's a look at some of the most gerrymandered Republican states:
The GOP got outright House majorities despite statewide losses in Michigan, Ohio, Pennsylvania, Virginia, and Wisconsin, roughly getting twice as many seats as their statewide vote would suggest. The Democrats didn't manage that anywhere in the few states where they control the full redistricting process:
Democrats squeezed out modest advantages, particularly compared to the Republican gerrymandering efforts above. Meanwhile, non-partisan efforts produced the most equitable results:
Republicans lost the House vote yet managed a significant House majority. Given demographic changes, it's increasingly difficult for Republicans to win absent 1) calamitously low Democratic turnout, like 2010, or 2) rigging the system to provide systemic advantages. Democrats need to work toward eliminating both of those.
In case you are wondering, the very same gerrymandering that created those Republican-heavy districts also created heavily gerrymandered state legislative districts. So we've got quite the hole to dig out of.