Pop quiz! Who is the worst president in U.S. history when it comes to medical marijuana?

According to the Marijuana Policy Project, the answer is Barack Obama.

And the group accuses the president of pulling a bait-and-switch: “During his run for the presidency, Barack Obama instilled hope in medical marijuana supporters by pledging to respect state laws on the matter,” writes Rob Kampia, the MPP’s executive director. “And for the first two years of his term, he was generally faithful to his promise. Yet suddenly, and with no logical explanation, over the past eight months he has become arguably the worst president in U.S. history regarding medical marijuana.”

Among the administration’s recent moves:

• In September, the ATF sent a letter [PDF] to all federal firearms licensees instructing them that anyone who uses marijuana — even for medical purposes — cannot legally possess firearms or ammunition.  The letter goes even further to warn gun sellers:

[Federal law] makes it unlawful for any person to sell or otherwise dispose of any firearm or ammunition knowing or having reasonable cause to believe that such a person is an unlawful user of or addicted to a controlled substance.

• Last week, the IRS ruled that medical marijuana dispensaries cannot deduct standard business expenses — such as rent, employee health insurance, security, licensing fees or payroll — from their tax returns, in what observers say could be a crushing blow to the industry.

• And on Friday, federal authorities in California — the country’s largest marijuana market — launched a coordinated, statewide offensive against medical marijuana dispensaries, announcing a series of civil forfeiture lawsuits and warning at least 16 to shut down or face criminal charges and confiscation of property.

Prosecutors said the most recent crackdown was aimed at dispensaries that use medical marijuana laws to cover up illegal drug trafficking. “California’s laws have been hijacked by people who are in this to get rich and don’t care at all about sick people,” said Melinda Haag, the U.S. attorney in San Francisco. And the MPP’s Kampia notes that the raid last week did not target any state-licensed businesses.

But at the heart of the broader crackdown is a battle between state and federal laws that will likely continue to play out.

Sixteen 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.

Bonus: Are newspapers and radio stations that advertise dispensaries the next target? California Watch’s Michael Montgomery speaks to one U.S. attorney in California who says the ads are illegal — and she plans to act.