CNN

The recent US Supreme Court decision changing voter registration rules in Arizona has voting rights advocates anxious about how the justices will approach emergency election appeals in the runup to the November election.

Not only was the Arizona ruling a missed opportunity for the justices to explain when they will engage in election and voting cases, experts say it has also heightened concern that the court is unevenly applying a murky legal principle intended to reduce chaos rather than add to it.

The “Purcell principle,” rooted in a 2006 Supreme Court decision, warns federal courts to avoid making last minute changes to the status quo of voting rules before an election.