How the 'independent state legislature' theory, now rejected by SCOTUS, fueled chaos in 2020 and could influence 2024

The Supreme Court on Tuesday rejected a controversial legal theory that would've given partisan state lawmakers nearly unchecked power over US elections.

Former President Donald Trump and his staunch allies used the now-rejected "independent state legislature" theory to justify their attempts to overturn the 2020 election. And many Trump critics warned that, without action from the Supreme Court, these same vulnerabilities would threaten the 2024 election.

In a case about North Carolina redistricting, the Supreme Court ruled that state courts and other state entities can review laws passed by state legislatures setting rules for federal elections. The court's majority - a coalition of three conservatives with the three-justice liberal bloc - rejected the GOP-backed theory that elected politicians have unreviewable authority to set election rules.

One of the reasons Republicans might want to shift power to state legislatures is because their party has a structural advantage on that front. Republicans currently control the legislatures in four states that Joe Biden carried in 2020 - Georgia, Arizona, Wisconsin and New Hampshire - and they control two additional statehouses in the battleground states of North Carolina and Florida.

States across the country adjusted their election rules in 2020, while the Covid-19 pandemic was raging and before vaccines were available. The changes included adding dropboxes in populated areas and easing the rules for when mail-in ballots can be accepted, among other things.

Many of these tweaks were implemented by state courts, governors, secretaries of state and other state election administrators. But according to the "independent state legislature" theory, these rule changes were illegal, because they didn't come directly from the state legislature.

This is what formed the basis of many of Trump's attempts to overturn the 2020 election.

Trump allies, like right-wing lawyer John Eastman, Texas Attorney General Ken Paxton and Missouri Sen. Josh Hawley, used this theory to argue Biden's victories in key states were illegitimate because they "unlawfully" conducted elections or "failed to follow their own laws."

This legal theory fueled their unsuccessful lawsuits seeking to nullify millions of votes, and their attempt to reject Biden's electors when Congress tallied the electoral votes on January 6, 2021.

Still, after the 2020 debacle, conservative legal figures kept up the fight, perhaps with an eye toward 2024. Top Republicans, including Trump and House GOP leaders, continued to peddle the theory. Eastman filed an amicus brief with the Supreme Court in the North Carolina case, urging the justices to give state legislatures full control over elections.

"Federal courts overwhelmingly rejected those Republican arguments before and after the 2020 elections, and the Supreme Court today put the issue to bed," said R. Stanton Jones, a lawyer who argued against the theory when the case was before the North Carolina Supreme Court.

The high court's ruling will have a significant impact on the 2024 presidential election, because it closes off some legal pathways for Trump to once again undermine the electoral process.

For starters, there is now Supreme Court precedent rejecting some of the more maximalist but unsettled theories that have been championed by Eastman and other GOP lawyers. (Never mind the fact that amid the 2020 chaos, even Eastman admitted that his harebrained legal proposals would be unanimously rejected by the Supreme Court, as CNN recently reported.)

But the somewhat limited ruling leaves plenty of avenues for future election-related challenges, regarding how districts are drawn, the deadlines for mail-in ballots, and other key questions.

Legal scholars observed Tuesday that the majority opinion, written by Chief Justice John Roberts, specifically said federal courts have "a duty to exercise judicial review" over state court decisions that influence federal elections. But the majority opinion didn't set the ground rules.

"By not setting a clear standard for when state courts would go too far in the future, the decision leaves open a number of questions that will have to be resolved in future election-related disputes," said Steve Vladeck, a CNN Supreme Court analyst and professor at the University of Texas School of Law.

Indeed, Adam Kincaid, who leads a national GOP redistricting group, said in a statement that Tuesday's ruling "should serve as a warning to state courts inclined to reach beyond the constitutional bounds of judicial review," signaling that there are plenty of lawsuits to come.

CNN's Ariane de Vogue contributed to this report.