Republican gains in statehouses during Obama's first midterm produced a priceless advantage when House districts were redrawn after the 2010 census, and AP has quantified that in a fascinating way:

"Republicans [last year] won as many as 22 additional U.S. House seats over what would have been expected based on the average vote share in congressional districts across the country. That helped provide the GOP with a comfortable majority over Democrats instead of a narrow one."

That's the most striking finding of a project the AP has been planning with members for weeks, "Redrawing America: Imbalance of Power ... how gerrymandering benefited GOP in 2016," by David Lieb:

• "The AP scrutinized the outcomes of all 435 U.S. House races and about 4,700 state House and Assembly seats up for election last year."
• "The analysis found four times as many states with Republican-skewed state House or Assembly districts than Democratic ones. Among the two dozen most populated states that determine the vast majority of Congress, there were nearly three times as many with Republican-tilted U.S. House districts."
• "Traditional battlegrounds such as Michigan, North Carolina, Pennsylvania, Wisconsin, Florida and Virginia were among those with significant Republican advantages in their U.S. or state House races. All had districts drawn by Republicans after the last Census in 2010."
• "[E]ven if Democrats had turned out in larger numbers, their chances of substantial legislative gains were limited by gerrymandering."