In suing to stop marijuana legalization in Colorado, two neighboring states have embarked down an arcane legal pathway that could take years to reach a conclusion, legal scholars say.

Nebraska and Oklahoma last week asked the U.S. Supreme Court to toss out portions of Colorado’s pot legalization law. The states contend that Colorado’s law — and especially Colorado’s licensing and regulation of marijuana stores — violates the U.S. Constitution’s Supremacy Clause, which says the federal law reigns when state and federal laws are in irreconcilable conflict.

But the infrequency with which states sue other states makes it impossible to predict how the case will play out. The nation’s highest court could shut down Colorado’s burgeoning recreational marijuana industry, calling into question the similar industry in Washington state and the planned industries in Alaska and Oregon. Or it could refuse to hear the case, leaving Colorado’s law intact.

“The court rarely grants such cases, perhaps one or two a year,” James Pfander, a law professor at Northwestern University, wrote in an e-mail.

By U.S. law, states suing other states must file their cases directly to the U.S. Supreme Court. Because they originate with the Supreme Court, such cases are known as “original proceedings.”

Only two such cases were filed at the Supreme Court in the court’s October 2013 term — out of more than 7,300 filed in total. According to the Federal Judicial Center, fewer than 140 such cases have been filed since 1960, and the court declined to hear roughly half of those cases.

In contrast to typical lawsuits, which are guaranteed at least an initial review by a judge, the first step for any original case is to persuade the Supreme Court to hear it. That’s why the Nebraska- Oklahoma filing last week contained three separate elements: a motion asking the Supreme Court for permission to file the lawsuit; a complaint explaining the lawsuit; and a brief providing the legal basis for the lawsuit.

Colorado officials have 60 days to respond, and it’s possible a lawyer for the U.S. government also will weigh in. Colorado Attorney General John Suthers, who last week met with several lawyers in his office to discuss Colorado’s response, declined an interview request.

If the Supreme Court agrees to take the case, it will likely appoint a “special master” — someone who will act like a trial judge, freeing the Supreme Court justices from the more tedious tasks of case management.

Even among original proceedings, the Nebraska-Oklahoma case is unusual. Most original cases involve disputes over borders or water rights. As legal blogger Kent Scheidegger noted last week, “It’s a pretty wet docket, and generally quite boring.”

Those cases often take years or decades to resolve. The Supreme Court this month issued an order in an original case that it first decided in 1947. A 2008 special master report on a water lawsuit involving Kansas, Nebraska and Colorado noted that the case’s litigation history stretched back more than a century.

Legal scholars have generally been skeptical of the Nebraska-Oklahoma pot lawsuit. But the states’ argument may have an ally on the Supreme Court.

Earlier this year, after a speech in Boulder, Justice Antonin Scalia was asked whether Colorado’s legalization of marijuana infringes on the federal government’s authority. Scalia sidestepped the question, noting that the issue might one day come before the Supreme Court. But then he added, “The Constitution contains something called the Supremacy Clause.”

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/johningold