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The Supreme Court gave good news Monday night by doing nothing. The Court turned away a request by the Republican National Committee to prevent Pennsylvania from counting mail-in ballots that are received up to three days after Election Day. That means that more people’s legitimate votes will be counted, which is an unqualified win for democracy.

But that good news may not last. The state of play after Election Day will involve the confluence of state and federal constitutional law, a hotly contested Supreme Court case from just five years ago, another one from over a century ago, and the unprecedented possibility of a new Supreme Court Justice being seated days before a presidential election on which the Court may have to rule just weeks later.

Until this fall, if a Pennsylvania voter mailed her ballot on the day before Election Day but it didn’t arrive at the election board until the day after Election Day, that vote wouldn’t be counted. In light of the extraordinary circumstances surrounding an election in a pandemic with millions of mail-in ballots expected to be cast through a hobbled Postal Service, the Pennsylvania Supreme Court extended the deadline by three days. It based its decision on the Free and Equal Elections Clause of the Pennsylvania Constitution: “Elections shall be free and equal; and no power, civil or military, shall at any time interfere to prevent the free exercise of the right to suffrage.” (By the way, that sounds like a great addition to our federal Constitution.)

But even though the Court denied the Republicans’ request to put the state court decision on hold, four Justices said (without elaborating) that they would have gone the other way: Justices Thomas, Alito, Gorsuch, and Kavanaugh. Which means that the Court was deadlocked at 4-4. Which means, in turn, that the addition of a new Justice Barrett would be the deciding vote.

Monday night’s good news for democracy turns more ominous still because the Supreme Court’s inaction was not a final resolution. It just declined to overturn the Pennsylvania Supreme Court for now. Republicans can, and presumably will, ask the Court to step in again in the coming weeks so any mail-in ballots by Pennsylvania voters that were received after Election Day cannot be counted. By then, Judge Barrett might be seated and the conservatives on the Court might thus have the votes to hand the Republicans a victory after all. In light of the President’s unveiled hope and even explicit expectation that Judge Barrett would rule in his favor, it might seem like the fix is in.

The legal arguments the Court is likely to confront in this case will not be about the President, or the pandemic, or the Postal Service. They will be about the truly arcane relationship between state courts and state legislatures in presidential elections: does the Constitution say that the state legislature alone, unfettered by the state courts or even a state’s constitution, gets to decide how to hold presidential elections?

Shockingly, the Constitution does not guarantee the American public the right to vote for president. Instead, the Constitution says that the presidential electors are appointed by each state “in such Manner as the Legislature thereof may direct.” Every state now chooses its electors by popular election, but that wasn’t true until the middle of the 19th century. In other words, your right to vote for president was bestowed by your state legislature.

That doesn’t mean the state legislature can hold the presidential election in whatever way it wants. If a state legislature tried to limit the vote to people from a particular religion, that would undoubtedly violate the First Amendment. Or if it tried to limit the vote to people of a particular race, that would violate the Fourteenth and Fifteenth Amendments. The harder question in the Pennsylvania case is whether a state legislature is constrained by its own state constitution in deciding how to hold the election.

The answer depends on how the Court treats one case from 1892 and another case from 2015. The 1892 case, McPherson v. Blacker, says that the Constitution grants state legislatures “plenary control” over the manner in which presidential electors are selected. The Supreme Court seemed to endorse some version of that principal in Bush v. Gore. Smart commentators have called this Republicans’ “boldest and perhaps most dangerous” argument. They’re right to be concerned. After all, the text of the Constitution explicitly mentions the state legislature, not the state courts or the state constitution.

But the McPherson “plenary power” principle runs headlong into a 2015 case about congressional redistricting in Arizona. The Constitution assigns the authority to “prescribe” the “Manner” of elections for the Senators and Representatives from each state to “the Legislature thereof.” That should sound familiar; it’s precisely the same phrase in the Constitution about presidential electors. And the Supreme Court said in the Arizona case: “Nothing in the [Constitution] instructs, nor has this Court ever held, that a state legislature may prescribe regulations on the time, place, and manner of holding federal elections in defiance of provisions of the State’s constitution.”

So that should settle it, right? Just five years ago, the Supreme Court said that under a virtually identical Constitutional provision, state legislatures have to follow state constitutions in setting congressional elections. It stands to reason that Pennsylvania’s state constitution can also constrain how its state legislature sets a presidential election. And according to the Pennsylvania Supreme Court, its state constitution requires the extension of the mail-in ballot deadline.

Not so fast. There is one last turn in this story. The Supreme Court decided the Arizona case 5-4, with Justice Ginsburg writing for the majority and Chief Justice Roberts issuing a blistering dissent — he who voted, for now, with the liberals in the Pennsylvania case Monday night. Since then, Justices Scalia, Kennedy, and now Ginsburg have been replaced by Justices Gorsuch, Kavanaugh, and potentially a new Justice Barrett. That change could be decisive.

A very close presidential election that comes down to late-arriving mail-in ballots in Pennsylvania could thus turn on whether the Supreme Court revisits the Arizona decision on this obscure issue of state and federal constitutional law with a decidedly more conservative lineup of Justices. And so long before Roe v. Wade returns to the Court, Judge Barrett’s views on precedent could decide our democracy’s fate.

Matthew A. Seligman is Special Counsel for Election Integrity with the Campaign Legal Center. He is currently teaching a seminar on disputed presidential elections at Harvard Law School.