Yesterday, the Supreme Court took a step in a high-profile case to preserve democratic checks and balances.

The details of the case, Moore v. Harper, can sound technical. But it is simply about which officials can oversee federal elections. The petitioner, a North Carolina lawmaker, had sought to radically reshape how federal elections are conducted. The court ruled that state legislatures do not have unchecked power over elections and that other government officials can question and overturn their decisions.

Chief Justice John Roberts and two other conservatives joined the court’s three liberals in the ruling. The Constitution, Roberts wrote, “does not exempt state legislatures from the ordinary constraints imposed by state law.”

Why does the ruling matter? Because it makes it more difficult for partisan state legislatures to flout the law or norms to keep their party in power, at a time when most legislatures have one-party supermajorities. Under the Supreme Court ruling, other officials can step in if they feel state lawmakers went too far in rewriting election law. The decision, then, could influence which party controls Congress in the future.

The facts of the Supreme Court case help clarify the ruling’s potential impact. They offer a real-world example of how branches of government can check each other — in this case, courts over legislatures.

In 2021, lawmakers in North Carolina drew a voting map that would have likely given Republicans 10 of its congressional seats and Democrats four — a lopsided result in a state that is close to evenly divided politically. The state’s Supreme Court initially rejected the redrawn map. And in November, the state held elections with a map drawn by experts appointed by a state court. The result: a congressional delegation evenly divided between Democrats and Republicans, mirroring the state’s actual political makeup.

In short: Without the court, Republicans would have drawn a map that heavily favored their party. With the court’s intervention, the congressional map accurately reflected the state.

Last year, Republican lawmakers took the issue to the U.S. Supreme Court. They invoked what is known as the independent state legislature theory. They claimed the Constitution’s Elections Clause empowers only state legislatures, not courts, to set rules for congressional elections, including redrawing district lines.

The other side, made up of Democratic voters and advocacy organizations, argued that state legislatures’ election decisions should have many checks: the courts, governors, independent commissions and other officials empowered under the law.

In yesterday’s decision, the Supreme Court rejected the independent state legislature theory. Roberts cited the long history of state courts reviewing and striking down state laws, and argued that nothing exempts election laws from such judicial review.

“The extreme version of the theory was soundly rejected, and by a six-justice majority,” my colleague Adam Liptak, who covers the Supreme Court, told me. “It would seem to be quite dead.”

It is the second time this month that Roberts and another conservative justice, Brett Kavanaugh, have joined the liberals in an election case. They also did so in a case over Alabama’s congressional map. The two rulings suggest there may be a moderate wing on voting issues in an otherwise conservative Supreme Court.

Three of the court’s conservatives dissented yesterday, largely on procedural grounds. After Republican lawmakers appealed the case to the U.S. Supreme Court last year, conservatives took over the North Carolina Supreme Court and reversed its previous ruling on the state’s congressional map. That eliminated the need for a U.S. Supreme Court ruling on the issue, the three justices argued. The court’s bipartisan majority disagreed.

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