(CNN) -- The federal government will continue to enforce federal marijuana laws in California even if the state legalizes pot through a ballot initiative next month, Attorney General Eric Holder said in a letter obtained Friday by CNN.
Holder said the Justice Department would continue to "vigorously enforce" the federal Controlled Substances Act "against those individuals and organizations that possess, manufacture, or distribute marijuana for recreational use, even if such activities are permitted under state law," in a letter to former administrators of the Drug Enforcement Administration.
"Regardless of the passage of this or similar legislation, the Department of Justice will remain firmly committed to enforcing the Controlled Substances ACT ("CSA") in all states," said the letter, dated Wednesday.
California's Proposition 19, which will appear on next month's ballot, would legalize recreational marijuana use in the state. Supporters say it would raise revenue and cut the costs of enforcement.
An analysis by the California attorney general's office says the state would reap the benefits of "additional revenues from taxes, assessments, and fees from marijuana-related activities allowed under this measure."
But Holder's letter said that "the Department of Justice strongly opposes Proposition 19."
"Prosecution of those who manufacture, distribute, or possess any illegal drugs -- including marijuana -- and the disruption of drug trafficking organizations is a core priority of the Department," the letter said.
Holder said that California's legalization of marijuana would throw up a major stumbling block to federal partnerships with state and local authorities around drug enforcement.
"The broad language of Proposition 19 that would prohibit state and local law enforcement from seizing marijuana that is in compliance with state law would provide a significant impediment to these joint efforts by law enforcement to target drug traffickers who frequently distribute marijuana alongside cocaine and other controlled substances," his letter said.
Holder's letter was a response to an August letter from 8 former directors of the U.S. Drug Enforcement Administration urging the White House to block Prop. 19 if it is approved next month.
The August 24 letter urged Holder to use the federal "supremacy clause" to pre-empt such lawmaking by state and local jurisdictions.
The Justice Department recently used such a preemption to persuade a court to strike down Arizona's new immigration law.
Holder's letter did not mention that option but said that "the Department is considering all available legal and policy options in the event Proposition 19 is enacted."
"We ... will give serious consideration to the information and authority you provide," Holder's letter continued, citing the advice of the former DEA directors.