marijuanaU.S. Attorney General Eric Holder says the federal government will enforce its marijuana laws in California even if the state’s voters approve a ballot measure to legalize the drug.

Holder says the Justice Department strongly opposes California’s Proposition 19 and remains firmly committed to enforcing the federal Controlled Substances Act (CSA) in all states.

He made the comments in a letter to former chiefs of the U.S. Drug Enforcement Administration. The letter was dated Wednesday.

“We will vigorously enforce the CSA against those individuals and organizations that possess, manufacture, or distribute marijuana for recreational use, even if such activities are permitted under state law,” Holder wrote.

He also said that legalizing recreational marijuana in California would be a “significant impediment” to the government’s joint efforts with state and local law enforcement to target drug traffickers, who often distribute marijuana alongside cocaine and other drugs. Holder said approval of the ballot measure would “significantly undermine” efforts to keep California communities safe.

If Proposition 19 passes in November, California would become the first state to legalize and regulate recreational pot use. Adults could possess up to an ounce of the drug.

California, like 13 other states and the District of Columbia, has legalized medical marijuana. In that realm, federal agents have focused on busting dispensaries they said were using medical marijuana as a front for traditional drug-dealing.

The Associated Press contributed to this report.