Six Colorado sheriffs sued their own governor in federal court on Thursday, arguing that state laws legalizing marijuana require them to violate federal law.

Joining the sheriffs in the lawsuit — the fourth to be filed against Colorado over marijuana legalization — are sheriffs and prosecutors from Nebraska and Kansas. The suit argues that federal law’s supremacy means Colorado cannot allow people to possess or use marijuana for recreational purposes and cannot license stores to sell it. The lawsuit does not challenge Colorado’s medical marijuana laws or stores.

“This suit is about one thing — the rule of law,” Larimer County Sheriff Justin Smith said in a news release. “The Colorado Constitution mandates that all elected officials, including sheriffs, swear an oath of office to uphold both the United States as well as the Colorado Constitutions.”

The suit names Gov. John Hickenlooper as the lone defendant. Colorado’s attorney general, Cynthia Coffman, will defend the state against the lawsuit, a spokeswoman for her said.

DOCUMENT: Read the recreational marijuana lawsuit

The lawsuit has the backing of national anti-legalization groups, and its nationwide ambitions were apparent in the choice of where to announce the suit. In a news conference Thursday morning at the National Press Club in Washington, D.C., Smith told reporters that Colorado’s passage of Amendment 64 created “a constitutional showdown” in the state. The amendment, which voters approved in 2012, made it legal for people over 21 in Colorado to possess limited amounts of marijuana and for licensed stores to sell marijuana.

“We think that what Colorado has done is illegal; we think it’s unconstitutional,” Scotts Bluff County, Neb., Sheriff Mark Overman, who is also a plaintiff in the suit, said at the news conference. “I believe, my fellow plaintiffs believe, that this case is going to have national ramifications. And, if we win, then we can reverse what looks like a surrender to the pro-marijuana crowd.”

The lawsuit follows another legal challenge filed in December by Nebraska and Oklahoma, which asked the U.S. Supreme Court to strike down the history-making law. Two lawsuits filed earlier this year by the anti-legalization group Safe Streets Alliance on behalf of several Colorado residents are also pending against the state over Amendment 64.

RELATED: Larimer Sheriff sues Colorado, but 54% of his county voted to legalize pot

Similar to the previous lawsuits, the new suit seeks to close all of Colorado’s recreational marijuana stores, but it also asks a federal judge to overturn Amendment 64’s protections for marijuana use and possession. The law enforcement officials’ suit claims that local officers are breaking federal law and international treaties when they find marijuana but don’t seize it.

At Thursday’s news conference, Yuma County Sheriff Chad Day said Colorado’s marijuana laws now prohibit officers from seizing marijuana in certain circumstances — putting officers, he said, in “an untenable position.”

“By supporting and upholding one, we’re violating the other,” he said. “I’m excited to, at the very least, get an answer from the courts.”

Sam Kamin, a law professor at the University of Denver, was skeptical of the sheriffs’ argument. He said no law requires local officers to act as de facto federal drug agents.

“Of the four (lawsuits), this is the one with the least merit,” Kamin said. “They have targeted not just the (marijuana store) regulation piece but they’re also essentially saying Colorado can’t legalize marijuana. No one has ever gone that far.”

Colorado plaintiffs in the latest lawsuit include Smith, Day, Elbert County Sheriff Shayne Heap, Hinsdale County Sheriff Ronald Bruce, Kiowa County Sheriff Casey Sheridan and Delta County Sheriff Frederick McKee.

Of the six Colorado sheriffs involved in the lawsuit, only Smith is from a county that approved Amendment 64, with 55 percent of 86,410 voters approving the measure. The measure was approved in 33 of 64 Colorado counties by a 55 percent margin.

Also named as plaintiffs are three Nebraska sheriffs: Adam Hayward of Deuel County, Mark Overman of Scotts Bluff County and John Jenson of Cheyenne County. Cheyenne County Attorney Paul Schaub of Nebraska and Sheriff Charles Moser of Sherman, Wallace and Greeley counties in Kansas also signed the document.

As in the lawsuit filed by Nebraska and Oklahoma, the law enforcement officials from Nebraska and Kansas are arguing in the new lawsuit that marijuana flowing out of Colorado post-legalization has led to more arrests and strained their budgets.

“It’s actually taking money out of Sherman County taxpayers’ pockets,” Burton Pianalto, the sheriff in Sherman County, Kan., said at Thursday’s news conference.

In January, nine out of the 46 people booked into Sherman County’s jail were suspected of possessing or distributing hallucinogenic drugs, the category under which marijuana is classified in Kansas criminal law, according to an online booking log. Two out of the 15 inmates listed on Thursday as being currently housed in the county jail face charges related to hallucinogenic drugs.

Colorado officials have not yet responded to any of the pending lawsuits. Previously, state attorneys have argued that Colorado’s more permissive marijuana laws — both for medical and recreational use and sales — do not amount to legalization of marijuana. Instead, they contend that marijuana remains illegal in Colorado, aside from the specific exemptions allowed under the medical and recreational marijuana laws.

When Nebraska and Oklahoma filed their lawsuit against Colorado last year, then-Colorado Attorney General John Suthers said in a statement that , “it appears the plaintiffs’ primary grievance stems from non-enforcement of federal laws regarding marijuana, as opposed to choices made by the voters of Colorado.”

Mason Tvert, one of the lead proponents of Amendment 64, said if the lawsuits are successful in striking down the measures, it would lead to “a system of marijuana chaos.”

“This is just another case of the Arrest and Prosecution Industry teaming up with marijuana prohibition groups to roll back the progress that has been made in Colorado,” he said in a statement Thursday.

U.S. Rep. Jared Polis said in a statement that the U. S. Department of Justice and President Obama have made clear that they will not interfere with states whose citizens voted for recreational marijuana use.

“This lawsuit is a silly attempt to circumvent the will of Colorado voters and is a waste of time…,” Polis’ statement says. “Unfortunately, these frivolous lawsuits will likely continue until we pass my bipartisan Regulate Marijuana Like Alcohol Act and finally end the outdated federal prohibition of marijuana.”

Kirk Mitchell: 303-954-1206, denverpost.com/coldcases or twitter.com/kirkmitchell