What Bender learns, via some quite simple research, is the reason why this is true:
The districts where Democrats have the best shot to win Republican-held seats show some of the smallest gaps between rich and poor in the U.S., an indication of just how hard it will be for their message to take hold with voters.Dems already represent most districts which experience vast income inequality. Thus they are "preaching to the choir," rousing the base. That's only useful IF it results in increased Dem turnout in statewide U.S. Senate races, not a probable outcome.
Of the 100 congressional districts ranked as having the greatest gap between rich and poor, not one is held by a Republican whose seat is considered up for grabs this November.
You might also enjoy Megan McArdle's take on Bender's findings, also from Bloomberg.