SPRINGFIELD, Illinois (Reuters) - The Illinois General Assembly on Wednesday put off a vote to legalize marijuana use for medical purposes because the measure lacked the support for approval, its chief sponsor said.

Democratic Representative Lou Lang did not request a vote on his proposal because he did not want it to fail.

"He didn't call it because he was short of the votes," said Lang's spokeswoman, Beth Hamilton. Lang had earlier predicted the measure would pass if a few undecided members shifted to support.

The proposal for a three-year pilot program would make Illinois the second most populous state in the nation after California to allow medical marijuana. Nineteen states and the District of Columbia have legalized medical marijuana. Colorado and Washington state voters decided on November 6 to allow recreational use of cannabis.

Lang said he could try again to pass the proposal when the Illinois legislature meets in early December.

The Illinois bill would be the most restrictive in the country, according to Lang.

Some Republicans in the Illinois House said they opposed legalizing medical marijuana because it could be a "gateway drug" to abuse of other illegal substances. Others said they were not convinced that the benefits of smoking marijuana for certain medical conditions outweighed the potential negative consequences.

Under the Illinois bill, patients would have to be diagnosed with one of 30 debilitating medical conditions, register with the Department of Public Health and have written certification from their physician. Patients would be limited to no more than 2.5 ounces (70 grams) of marijuana every two weeks.

Under U.S. federal law, marijuana is considered an addictive substance and distribution is a federal offense. Federal law prohibits physicians from writing prescriptions, so many have issued "referrals" or "recommendations." The administration of President Barack Obama has discouraged federal prosecutors from pursuing people who distribute marijuana for medical purposes under state laws.

(Editing by Greg McCune, Mohammad Zargham and Leslie Adler)