By Kurtis Lee, Los Angeles Times (TNS)

PUEBLO WEST, Colo. — On the front door of the 20,000-square-foot marijuana dispensary here is a laminated sign warning every customer: “It is illegal to sell or transport marijuana to another state.”

“And you can guarantee people read it,” said Rick Hooper, general manager of the Spot 420 in this barren part of southern Colorado. “We make it very, very clear that this is the law here.”

Whether people obey is an entirely different question, and some neighboring states don’t think a warning sign is enough.

A border war has broken out between Colorado, where recreational pot is legal, and its neighbors, Nebraska and Oklahoma, where it is not.

In December, the attorneys general of Nebraska and Oklahoma sued to stop what they say is a steady flow of marijuana across the Colorado state line. Kansas is considering joining as well.

The lawsuit, filed directly to the U.S. Supreme Court, seeks to strike down Colorado’s law legalizing recreational marijuana. It argues that Colorado’s statute conflicts with federal drug laws, which consider marijuana illegal, even in small amounts.

“Left unchallenged, I am confident Colorado’s law will cause long-term harm to Nebraska families,” the state’s new attorney general, Republican Doug Peterson, wrote in an open letter last week. “It is incumbent on Nebraska to take action.”

Coloradans, however, are bristling that its staunchly conservative neighbors are trying to impose their will on the “open-minded voters” of this centrist state.

“They can’t force their convictions onto Coloradans,” Hooper said.

Colorado’s marijuana law was approved by voters in 2012. It allows the sale and possession of up to an ounce of marijuana for recreational use for anyone 21 and over with a valid driver’s license.

Shortly after the new law took effect, the U.S. Justice Department outlined its enforcement priorities, saying it would not interfere with Colorado’s legal pot operations but would instead focus on, among other things, preventing marijuana from crossing state lines.

Oklahoma’s Bureau of Narcotics and Dangerous Drugs said it had seen more “high-potency” marijuana arriving from its neighbor. Mark Woodward, an agency spokesman, said there had been about a dozen cases in the past year.

“Whether it’s people driving to Colorado and bringing it back, or mailing it through the Postal Service, it’s getting here,” he said. “This is marijuana with very high concentrations of THC, very strong stuff.”

Some police in Colorado agree it’s not difficult to get marijuana across state lines. “People can buy legal marijuana, take it out of its packaging, put it in a plastic bag, and there’s no telling if it’s legal or where it came from,” said Marc Vasquez, the Erie, Colo., police chief.

Colorado recently launched a $5.7-million ad campaign to make it clear to everyone — especially out-of-state visitors — what the rules are. Taking pot out of the state is a felony and a federal violation.

But the success of the campaign is debatable, given the ease of driving across state lines.

In Denver last fall, the police and the U.S. Drug Enforcement Administration raided several growing facilities that officials said were producing marijuana for out-of-state sellers.

But most dispensaries are not partaking in illegal activity, Hooper said. “We follow the rules, very strictly. Why jeopardize this movement?”

Nebraska and Oklahoma’s lawsuit argues that Colorado cannot pass statutes that conflict with federal drug laws. It is a violation of the U.S. Constitution, which maintains that federal law is the “supreme law of the land,” according to the suit.

In addition, the suit argues that Oklahoma and Nebraska will suffer in the long term because of increased costs from arrests, the seizure of contraband, the transfer of prisoners and other problems associated with marijuana crossing state lines.

Legal experts have mostly scoffed at the suit.

“This is a very weak claim. Their real beef is with the federal government for not enforcing the federal drug laws,” said Georgetown law professor Randy Barnett, who has argued a marijuana case before the U.S. Supreme Court. “It is not up to the states to sue each other when the federal government is not enforcing the law.”

The Supreme Court already has found that states have no duty to enforce federal law.

Oklahoma Attorney General. Scott Pruitt, a Republican, and Nebraska’s Peterson declined to comment for this story.

Critics of the lawsuit largely see it as political grandstanding by the attorneys general to their conservative constituencies.

But even among conservatives, there are complaints.

Last month, a number of GOP legislators, led by Oklahoma state Rep. Mike Ritze, sent a three- letter to Pruitt asking him to drop the suit because of its assault on the right of a state to pass its own laws.

We “do not feel that attempting to undermine the sovereignty of a neighboring state using the federal courts, even if inadvertently, is a wise use of Oklahoma’s limited state resources,” the letter said.

Peterson, in his open letter, stressed that he had no intention of giving up the suit.

“Nebraska has only one real choice, to uphold the law that exists for the protection of the public and well-being of Nebraska’s families,” he wrote. “We must not subject our youth to such a costly social experiment.”

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(Times staff writer David G. Savage in Washington contributed to this report.)

Photo: Matt Beizaee is an employee of the Spot 420, a recreational marijuana dispensary in Pueblo West, Colo. (Kurtis Lee/Los Angeles Times/TNS)