On October 23, the Pennsylvania Supreme Court ruled that many voters who cast a mailed-in ballot improperly, thus rendering their vote “void,” should be allowed to cast a ballot on Election Day that will actually be counted. Though it’s hard to pin down exactly how many voters will be impacted by this decision, it’s likely that thousands of Pennsylvanians will regain their ability to vote in the November election if the state supreme court’s decision remains in effect.

The case, which was known as Genser v. Butler County Board of Elections when it was decided by the state supreme court, involves a genuinely difficult legal question that required the state’s justices to resolve a conflict between two competing legal principles. It split them 4-3, with four Democrats forming the majority and two Republicans and one Democrat in dissent.

On Monday, the Republican Party asked the US Supreme Court to intervene and effectively disenfranchise the thousands of voters who will be allowed to vote if the state court’s Genser decision stands. That alone is a very big deal. If the 2024 presidential election is very close, as is expected, and if victory comes down to which candidate wins Pennsylvania, a US Supreme Court decision in favor of the GOP could flip the winner of this election from Kamala Harris to Donald Trump. (Although more Republicans are expected to vote by mail than in 2020, Democrats likely still have an advantage with this cohort.)

But this case could also have massive implications for US democracy that extend far beyond the 2024 election, even if Harris wins in a landslide.

State supreme courts are supposed to be the final word on all questions of state law. The ultimate authority on the question in Genser has spoken, and the US Supreme Court ought to butt out. Nevertheless, the GOP argues that the high Court needs to get involved because of something known as the “independent state legislature doctrine” (ISLD), a chaotic legal theory that the Supreme Court has rejected many times over the last century.

If the justices accept the GOP’s arguments, they will introduce unprecedented chaos into US elections, putting in doubt which officials are responsible for election law — and even whether voters or their elected officials get to decide who wins in elections.

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The independent state legislature doctrine would destabilize the United States and threaten its security

In its strongest form, the ISLD claims that each state’s legislative branch has exclusive authority to determine how federal elections are conducted in that state. That means that a state governor cannot veto laws governing federal elections, even if the state constitution typically allows the governor to veto bills passed by the legislature. It means that states cannot alter their election laws through ballot initiatives or referendums. It means that state courts cannot enforce state constitutional provisions that protect the right to vote. And it even calls into question state court decisions interpreting state election laws.

In Genser, the Republican Party doesn’t go to any of those extremes. But they do claim that by interpreting Pennsylvania law to permit the impacted voters to cast a ballot on Election Day, the state supreme court robbed the state legislature of its supposedly exclusive authority to write election laws.

But even in its weaker forms, the ISLD is dangerous. The last time an independent state legislature doctrine case was before the Court, in Moore v. Harper (2023), an array of conservative luminaries and former top national security officials warned the justices to stay far, far away from the ISLD, lest they destabilize America’s entire system for choosing its leaders. That included a brief on behalf of retired admirals, generals, and service secretaries — some of whom held high-level political appointments in Republican administrations — who warned that the ISLD “undermines election integrity and exacerbates both domestic and foreign threats to national security.”

For the most part, the Court heeded these warnings in Moore. Although Chief Justice John Roberts and Justice Brett Kavanaugh had both expressed sympathy for the ISLD in the past, they largely reversed course in Moore — joining four of their colleagues in an opinion that mostly rejected this legal doctrine.

But Moore was not a total victory for democracy.

Historically, each state’s highest court has had the final word on all questions of state law. The US Supreme Court can intervene if a state violates federal law, or if a state supreme court hands down a decision that violates the US Constitution. But if a state supreme court interprets its own state’s law in a way that the federal justices don’t like, they are supposed to shut up and accept that they don’t have any power over a state’s internal self-governance.

Roberts’s majority opinion in Moore, however, included a cryptic line creating an exception to the rule that state courts have the final word on questions of state law. “[S]tate courts,” Roberts wrote, “may not transgress the ordinary bounds of judicial review such that they arrogate to themselves the power vested in state legislatures to regulate federal elections.” Moore, however, did not explain what, exactly, it means to “transgress the ordinary bounds of judicial review.”

As a practical matter, that means that Moore gave the US Supreme Court an unprecedented new power to overrule state election law decisions that at least five justices don’t like. It also means that Moore shifted a great deal of authority to the Republican Party — Republicans have a 6-3 majority on the US Supreme Court and Moore means that any five of these Republicans can reverse a state court decision that benefits Democrats.

The question in Genser is whether the Republican justices will use this self-given power, for the first time, to give Donald Trump an unfair advantage in Pennsylvania.

So where does the ISLD come from?

The independent state legislature doctrine is one of those very bad ideas that rears its head every few decades, only to get shut down when it reaches the Supreme Court. The Court first rejected the ISLD in Davis v. Hildebrant (1916), which upheld a provision of the Ohio constitution permitting the people of the state to veto state election laws via a popular referendum, even though that meant blocking a law enacted by the state’s legislative branch. The Court also handed down significant decisions rejecting the ISLD in 1932 and in 2015.

A likely reason why this doctrine refuses to stay dead is that it actually sounds plausible if you read the text of the Constitution without doing any legal or historical research. One provision of the Constitution states that “the times, places and manner of holding elections for Senators and Representatives, shall be prescribed in each state by the legislature thereof.” Meanwhile, another provision says that presidential elections shall also be conducted in a way determined by the state legislature.

So the argument for the ISLD is deceptively simple: The Constitution states that the rules governing federal elections are set by a state’s “legislature.” Governors, state courts, and state constitutions are not part of the legislative branch of government. Therefore they have no say in how federal elections are conducted.

The problem with this argument is that it is anachronistic. When the Constitution was drafted, the concept of a popularly elected legislative body was a relatively new innovation, and the word “legislature” did not always refer to a body, like the US Congress, which was defined as the “legislative” branch of government. Instead, it meant, in the words of one 1828 dictionary, “the body of men in a state or kingdom, invested with power to make and repeal laws.”

Or, as the Supreme Court held in Arizona State Legislature v. Arizona Independent Redistricting Commission (2015), the word “legislature” refers to “the power that makes laws.” That power can be vested in a congress or parliament. But it can also be partially vested in a state governor (who has the power to veto laws), in a state supreme court (which has the power to interpret laws), or in the people of a state as a whole (who may have the power to make laws via ballot initiatives or referendums).

Proponents of the ISLD, in other words, aren’t actually trying to preserve a separation of powers that existed when the Constitution was written. They want to create an entirely novel regime that upends existing state constitutions and gives unprecedented power to a Republican Supreme Court.

What are the immediate stakes in Genser?

Even setting aside the long-term stakes in this case, Genser is a big deal solely because it could change the outcome of the 2024 presidential election — although the likelihood of such an outcome is difficult to predict because it turns on two unknown variables: 1) How much of an advantage will Democrats have over Republicans among Pennsylvanians who vote by mail? and 2) How many of these voters will be impacted by the Genser decision?

Genser arises out of a quirk of Pennsylvania law. In order to cast a mailed ballot, the voter must stuff their ballot inside two separate envelopes. The first, often referred to as the “outer envelope,” includes information that can identify the voter who cast a particular ballot. The second, known as the “secrecy envelope,” does not. Pennsylvania law, moreover, is unusually unforgiving to voters who do not properly stuff their ballot in this second envelope: In 2020, the state supreme court held that a ballot that is not properly stuffed is “void.”

The plaintiffs in Genser are two voters who previously attempted to vote by mail, but their ballots were canceled because they didn’t properly stuff them. The state then sent them an email informing them that their ballots were voided and advising them that “you can go to your polling place on election day and cast a provisional ballot.” Yet, when these two voters did exactly that, the state reversed course and decided not to count their provisional ballots after all.

It turns out that the legal question of whether a Pennsylvania voter who casts a voided mailed ballot may then vote via a provisional ballot is unusually difficult. On the one hand, Pennsylvania law states that a provisional ballot “shall not be counted” if the same voter’s “mail-in ballot is timely received by a county board of elections.” So that statutory language is certainly helpful to the Republican Party’s position in Genser.

On the other hand, the state supreme court previously held that ballots cast without the secrecy envelope are “void.” As the same court explains in its Genser opinion, something that is “void” must be given “no legal effect.” So, under this reading of Pennsylvania law, a mailed ballot that is submitted without a secrecy envelope must be treated as if it did not exist — and thus it cannot prevent the same voter from casting a provisional ballot.

Is that a persuasive argument? The honest answer is that this case split the court 4-3 for a reason. Both possible interpretations of state law are plausible, and a majority of the state’s justices decided to go with the option that is more consistent with democratic principles.

It’s difficult to estimate exactly how many voters could be impacted by Genser. An MIT investigation into this question found that “about 6.4 percent of the 3,086 mail ballots cast in the 2019 general election in Philadelphia were rejected for not being enclosed within a secrecy envelope.” But during the 2020 election, “massive voter-education efforts” successfully reduced this number. A Philadelphia spokesperson claimed that only 1.1 percent of mailed ballots were rejected for this reason in 2020, and this number was much lower in other parts of the state.

Still, even if you assume that only a quarter of Pennsylvania voters will cast a mailed ballot in 2024, which is probably a low estimate, and that only 1 percent of those votes will not properly stuff their ballot in the secrecy envelope, that’s still a whole lot of voters. In 2020, nearly 7 million people cast a presidential vote in Pennsylvania. So, if a quarter of ballots were mailed in and 1 percent of those were invalidated, that’s more than 17,000 votes tossed out.

How many of those tossed-out votes would be for Harris is unclear. Though Republican participation in mail-in voting is higher than it was in 2020, the mere fact that the GOP has taken the position it’s taken in Genser suggests they expect mailed ballots to favor Democrats, as was the case in 2020.

Realistically, preventing these voters from having their votes counted will not change the result of the 2024 election unless the whole thing comes down to Pennsylvania — and unless Harris leads Trump by only a few thousand ballots. But, in that scenario, a victory for the Republican Party in Genser would change the course of American history.