Most U.S. attorneys general avoid partisan politics after leaving office. That’s not the case with Eric Holder. In 2016 Mr. Holder, who headed the Justice Department under President Obama from 2009-15, formed the National Democratic Redistricting Committee, ostensibly to combat gerrymandering. In reality, it’s a power play. The nation’s former chief law-enforcement officer wants to guarantee that Democrats control redistricting in 2021, when states redraw congressional and state legislative districts after next year’s census.