The Purcell principle, named for a dispute over an Arizona voter identification law taken to the court on an emergency basis in 2006, dictates that federal judges should generally refrain from causing confusion by changing voting rules in the lead-up to an election.

While the notion sounds simple enough, its application in practice can often be baffling. And just how to apply the idea of keeping election procedures stable in the midst of an extraordinary national health emergency like the coronavirus pandemic is far from clear.

“It’s being brought up in just about every case right now as we are getting closer to the election,” said Rick Hasen, a University of California at Irvine law professor who coined the term “Purcell principle” in a 2016 law review article. “But it’s not a hard-and-fast-rule, and it’s not well developed.”