The constitutional amendment approved by Missouri voters in November keeps those criteria. But it also requires a new nonpartisan state demographer to base state House and Senate districts on the votes cast in the previous three elections for president, governor and U.S. senator — races that are decided by voters statewide and are not affected by gerrymandering.

The districts must come as close as practical to achieving “partisan fairness” as measured by a formula called “the efficiency gap.”

That formula was created earlier this decade by Eric McGhee, a researcher at the nonpartisan Public Policy Institute of California, and University of Chicago law professor Nick Stephanopoulos. It compares the statewide share of the vote a party receives with the statewide percentage of seats it wins, taking into account a common political expectation: For each 1 percentage point gain in its statewide vote share, a party normally increases its seat share by 2 percentage points.

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