SAN FRANCISCO – Marijuana advocates are gathering signatures to get as many as three pot-legalization measures on the ballot in 2010 in California, setting up what could be a groundbreaking clash with the federal government over U.S. drug policy. At least one poll shows voters would support lifting the pot prohibition, which would make the state of more than 38 million the first in the nation to legalize marijuana. Such action would also send the state into a headlong conflict with the U.S. government while raising questions about how federal law enforcement could enforce its drug laws in the face...