By John Fritze, CNN(CNN) — President Donald Trump’s emergency appeal aimed at limiting the use of mail ballots is poised to test a controversial, 20-year-old Supreme Court principle that was intended to avoid chaos before elections. The “Purcell principle,” rooted in a 2006 Supreme Court decision, warns federal courts against making last-minute changes to voting rules. “This court has repeatedly stated that federal courts ordinarily should not enjoin a state’s election laws in the period close to an election,” Kavanaugh wrote. The state had also suspended voting in the House contests after the Supreme Court’s ruling in Louisiana v. Callais. The appeals court, tellingly, didn’t cite a single Supreme Court decision to back up that conclusion.