The Justice Department and federal banking regulators will help clear the way for financial institutions to transact business with the legitimate marijuana industry without fear of prosecution.
The Obama administration is facing criticism over its attempt to straddle the federal law that makes marijuana illegal and state laws that permit recreational use of the drug.
If it passes, Amendment 64 could have the U.S Justice Department suing the state of Colorado.
Federal officials on Thursday began a California-style crackdown on medical marijuana businesses in Colorado that targets those located near schools.
Shootings in suburban Aurora in March that left two police officers wounded and two suspects dead helped spark a summer-long crackdown in the Denver area on suspected armed gang members, federal and local law enforcement officials said Wednesday.
A high-ranking U.S. Justice Department official who wrote a memo saying state medical marijuana laws do not provide immunity from federal prosecution refused to say Wednesday whether a recent crackdown in California signals a shift in federal policy that may result in a crackdown in other states.