Voters in Missouri approved amending their state constitution this week with a subtle change that could spark a national legal fight over who is counted in voting districts.

In general, political mapmakers around the country have long drawn state legislative districts once a decade based on the total number of people living in an area as determined by the latest census results.

With the support of more than 51% of those who voted on the ballot measure, Republican state lawmakers have changed redistricting requirements so that going forward districts have to “be drawn on the basis of one person, one vote,” according to the newly-passed Amendment 3.

It’s not clear exactly how the phrase “one person, one vote” will be interpreted when the state’s voting maps are expected to be redrawn next year by either bipartisan commissions or judges from Missouri’s appellate courts. There is no requirement under Amendment 3 for Missouri to stop redistricting based on total population.But Missouri Solicitor General D. John Sauer was explicit about the state government’s interpretation during a court hearing in August.

“So ‘one person, one vote,’ the criteria is based on the number of actual eligible voters in a relevant district as opposed to an absolute population,” Sauer told state appeals court Judge Alok Ahuja during a hearing for a lawsuit over how the amendment was presented on election ballots, which did not go into any detail about how the redistricting criteria would be modified.

Sauer’s comment echoed the interpretation of Republican state Sen. Dan Hegeman of Cosby, Mo., a lead sponsor of the legislation that proposed the constitutional amendment who said during a Missouri senate floor debate in January that the phrase meant using a count of “people that are able to vote.”

Drawing state legislative districts based on the number of U.S. citizens old enough to vote would be a radical shift in political mapmaking. A GOP strategist concluded it “would be advantageous to Republicans and Non-Hispanic Whites.”

Opponents of Missouri’s Amendment 3 say the new criteria open the door to redistricting that does not take into account the political representation of children, noncitizens and other residents who are not eligible to vote.