Fifty percent of Americans now say they believe marijuana use should be made legal, the highest percentage to date, according to a nationwide Gallup survey released yesterday.

Support for legalization is up from 46 percent last year, with younger, politically liberal Americans more likely to favor legalization. According to the polling group:

When Gallup first asked about legalizing marijuana, in 1969, 12% of Americans favored it, while 84% were opposed. Support remained in the mid-20s in Gallup measures from the late 1970s to the mid-1990s, but has crept up since, passing 30% in 2000 and 40% in 2009 before reaching the 50% level in this year’s Oct. 6-9 annual Crime survey.

Those supporting legalization recently gained a powerful ally. Last Friday, California’s largest group of doctors announced that it supports legalization on the grounds that California’s medical marijuana law has created an “untenable situation for physicians: deciding whether to give patients a substance that is illegal under federal law.”

Dr. Donald Lyman, the physician who wrote the California Medical Association’s new policy told The Los Angeles Times that only after marijuana is legalized can the medical community undertake the research needed to determine whether the drug is useful for medical purposes.  The association is believed to be the first large medical association to call for legalization.

Still, there is significant resistance to legalization.  According to Gallup, 46 percent of Americans say marijuana use should remain illegal.  Those opposed to legalization often cite dangers associated with the drug’s use, and they may have just gotten some new ammunition, courtesy of researchers at Columbia University.  Their study, published in this month’s journal of Epidemiologic Reviews, found that marijuana use by drivers is associated with a “significantly increased risk of being involved in motor vehicle crashes.” According to the study, drivers who use the drug up three hours before getting behind the wheel are more than twice as likely to be involved in a crash.

To date, 16 states and the District of Columbia have passed medical marijuana laws, and more than a third of all states are experimenting with some form of legalization or decriminalization.