(Bloomberg) -- A US Supreme Court case brought by conservatives that would block state judges from undoing congressional district maps may hold a hidden advantage for Democrats. 

The plaintiffs are asking the high court to embrace the “independent state legislature” theory, which holds that the Constitution gives state lawmakers near total control of federal elections without judicial interference. That means a judge cannot throw out a highly partisan map drawn by the legislature, even if it waters down the vote of certain groups like ethnic or racial minorities.

But it would also undermine a citizen-led bulwark against partisan mapmaking in Democratic states like California, Colorado and Washington, leveling an advantage that Republicans used to win a narrow majority in the US House in November.

The high court heard arguments Wednesday in the case, with conservative justices questioning whether state supreme court judges can go too far when throwing out maps drawn up by a state legislature. 

Attorney Neal Katyal, who argued against the independent state legislature theory, said the case could open a “Pandora’s box,” undermining state election laws and leading to countless future court challenges to current court precedents.

Those precedents include a 2015 case in which the Supreme Court upheld an independent redistricting commission in Arizona that was challenged by state lawmakers on the grounds who argued that only they had the power to draw maps.

Caren Short, director of legal research for the League of Women Voters, which filed an amicus brief in the case, said that because many of the citizen-led commissions were created through ballot measures, rather than by state lawmakers themselves, they are vulnerable to legal challenges on those grounds. 

“Independent redistricting commissions are especially in jeopardy in this case,” she said. 

In recent years, 10 states have chosen — or have been directed by ballot initiatives — to use independent commissions to draw their maps for US House districts. Research has shown that commissions tend to draw fairer maps, with fewer districts that are packed with voters from one party, a common tactic for diluting the power of the opposing party.

If the court adopts even a narrow reading of the independent state legislature theory those citizen commissions are at risk, legal experts say.

Michael Li, a redistricting expert at the Brennan Center for Justice at New York University law school, said the court could end up empowering Democratic-led states to match the more aggressively partisan maps drawn up in Republican-led states like Texas and Florida.

“This is a classic case of ‘be careful what you wish for,’” he said of the conservative plaintiffs. 

Of the 10 states with commissions, President Joe Biden carried eight of them in the 2020 election: Arizona, California, Colorado, Hawaii, Michigan, New Jersey, Virginia and Washington state. The only two that lean toward Republicans were Idaho and Montana, which have just two House seats each.

Overall, Democrats won 80 seats in the 10 states with commissions in November, and Republicans 40, with one race still undecided. But even slightly more partisan maps in those states could be enough for Democrats to squeeze out enough seats to retake the majority, which the GOP currently holds by just eight seats.

In fact, an analysis by the consulting firm Redistricting Partners found that California lawmakers alone could have ensured Democratic control of the House if they had gerrymandered the map, instead of leaving it to a citizen-led nonpartisan commission.

Redistricting Partners owner Paul Mitchell said that it found that maps could be drawn that were “unfair but legally compliant” that would have ensured Republicans controlled as little as four seats in the California delegation, instead of the 12 they won in November.

Attorney David Thompson, who argued for the independent state legislative theory on Wednesday, repeatedly tried to narrow the theory so that it could still allow for things like a governor’s veto or a nonpartisan commission. 

Thompson, who represented the Republican speaker of the North Carolina House of Representatives, even said that his clients would not object to part of a Democratic bill that passed the US House earlier this year that would have mandated that all 50 states use independent commissions.

But Chief Justice John Roberts and Justice Amy Coney Barrett suggested that Thompson’s solution — to distinguish between “procedural” and “substantive” objections to a state legislature — was unworkable, echoing harsh skepticism from liberal justices Ketanji Brown Jackson and Sonia Sotomayor.

“Is your formalistic test just a way of trying to deal with our precedents or are you rooting that in the Constitution itself?” Barrett said. “Because you do have a problem with explaining why these procedural limitations are OK but substantive limitations are not.”

Apart from making one or two districts where all the minority party voters are based, state lawmakers can also draw up maps that break up communities to dilute their power, and even at times pit incumbents from the other party against each other, as happened to a number of Democratic members of Congress from Texas this year. 

Li said if California Democrats are put back in charge of redistricting, they could use similar tactics to target House Republican leader Kevin McCarthy, for example.

“Does Kevin McCarthy really want the California legislature drawing his seat?” he asked.

The case is Moore v. Harper, 21-1271.

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