President Donald Trump's emergency appeal to restrict mail-in voting is set to force the Supreme Court to grapple with a doctrine it constructed for one purpose — and may now be stretched to serve another entirely.

At the center of the dispute is the "Purcell principle," a two-decade-old rule cautioning federal courts against altering voting procedures close to an election, CNN reported. The doctrine traces to the Court's 2006 decision in Purcell v. Gonzalez, which involved a last-minute challenge to Arizona's voter identification law, and has since become one of the most frequently invoked tools in election litigation, appearing in disputes over ballot deadlines, voter roll purges, and ID requirements across numerous states.

In every prior invocation of Purcell, the Court has deployed the doctrine to prevent lower federal courts from overriding state election regulations — a safeguard grounded in federalism. As Notre Dame law professor Derek Muller explained, "Purcell has more weight when you are enjoining a state rule because of federalism concerns."

Trump's executive order, however, would upend that logic. It would require the U.S. Postal Service and the Department of Homeland Security to screen state voter rolls, withhold ballots from voters not listed on those rolls, and mandate tracking barcodes on ballot envelopes.

In other words, it is the federal government imposing new rules on states — and the states themselves are suing to block it. If the Supreme Court applies Purcell in this context, it would be wielding a doctrine designed to preserve state authority over elections to instead shield a federal encroachment into that very same authority.

"This mail voting executive order sits on the wrong side of the line," said Sophia Lin Lakin, director of the ACLU Voting Rights Project. "It's a federal actor imposing late-breaking changes on state election systems over the objections of the states that run them."

The Justice Department contends that the states sued prematurely, before the order was actually enforced. Should the Court agree, the states would need to refile their challenge — pushing the timeline closer to November's election.

At that point, the administration could invoke Purcell to argue that it is too close to Election Day for courts to intervene at all.

Solicitor General D. John Sauer has already begun laying the groundwork for that position, telling justices that the executive order "does not change anything at all about elections in any state" on its own.

While DOJ attorney Stephen Pezzi downplayed the likelihood of raising Purcell at a later stage, he stopped short of ruling it out entirely. "I, of course, can't and won't take an argument off the table about what the world will look like in August," he said.

A federal appeals court in Washington has already attempted to preempt that scenario. The panel ruled that blocking a late federal change actually preserves the "status quo" that Purcell is meant to protect — the opposite of what the administration might argue.

Notably, the panel cited no Supreme Court precedent for that conclusion, because none exists. The outcome could establish a new precedent for how — or whether — Purcell applies when the entity seeking to change election rules close to an election is the federal government rather than a state, a question the Court has never squarely addressed.