A second county has joined San Diego County supervisors' challenge to overturn California's 9-year-old medical marijuana law, the "Compassionate Use Act."

San Bernardino County supervisors voted in closed session Tuesday to file their own lawsuit challenging the voter-approved Compassionate Use Act, Proposition 215, and said Wednesday that they expected the courts would consolidate the two legal actions.

San Diego officials filed a lawsuit in U.S. District Court on Friday, arguing that Prop. 215 should be pre-empted by federal law, which says all marijuana use is illegal and that the drug has no medicinal value.

Bill Horn, chairman of the San Diego County Board of Supervisors, said Wednesday that he was not surprised that San Bernardino supervisors also decided to sue.

Meanwhile, officials from the state attorney general's office said they are still studying San Diego County's filing.

And American Civil Liberties Union officials said they expect federal officials to throw the county's lawsuit out before it is even argued.

San Diego County supervisors have been sharply criticized by local patients who use marijuana to ease pain, and marijuana advocacy groups.

On Tuesday, as San Bernardino supervisors were voting to file their own lawsuit, three groups in San Diego -- the ACLU of San Diego and Imperial Counties, as well as drug advocacy groups Americans for Safe Access and the Drug Policy Alliance -- said they would ask the court's permission to formally oppose San Diego County's suit.

ACLU officials said Wednesday that the courts have set a March 13 date to hear its arguments that it should be allowed to join the San Diego County lawsuit, and oppose it, on behalf of what they called unrepresented medical marijuana patients.

San Diego County's lawsuit was filed against the state of California and Sandra Shewry, director of the state's Department of Health Services, and asks the courts to stop the state from enforcing Prop. 215 -- essentially erasing it.

The county's lawsuit cites the U.S. Constitution's "supremacy clause," which states that the federal Constitution and federal law should be "supreme," as well as a 1961 U.S. treaty with 150 other nations outlawing marijuana as proof that Prop. 215 should be pre-empted by federal law.

Ruth Stringer, assistant county counsel for San Bernardino County, said the county expected to file its lawsuit in the same federal court soon.

"We'll be filing something similar to what San Diego filed," Stringer said. "They're the same issue."

But ACLU attorney Allen Hopper said he expects federal judges to throw out San Diego County's lawsuit before it is ever heard. Hopper and other ACLU officials said previous 9th Circuit Court of Appeals rulings have established that counties cannot sue their respective states over federal law.

The state attorney general's office agrees with that opinion, spokesman Nathan Barankin said.

But Horn and other San Diego County officials feel they have sound legal arguments that the courts will favor.

"I think we have the law on our side," he said. "If (the ACLU) is worried about going to court and the outcome, I think we're on the right track."

Contact staff writer Gig Conaughton at (760) 739-6696 or gconaughton@nctimes.com. To comment, go to nctimes.com.