A man purchases medical marijuana, the first legal sale, at Capital City Care in Washington, DC. The District of Columbia city council is to vote on decriminalizing pot.

The District of Columbia's city council will vote on Tuesday on whether to decriminalize marijuana in a move that could make smoking a joint a violation comparable to a parking ticket.

The bill is expected to pass the first of two votes required for legislation in the district, since eight of the council's 13 members have sponsored the measure, and Democratic Mayor Vincent Gray has said he favors it.

The decriminalization law could face scrutiny from Congress, which has constitutional oversight over the capital.

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Morgan Fox, a spokesman for the Marijuana Policy Project, a U.S. advocacy group, however, said there has been no sign the House of Representatives' Oversight Committee would oppose the measure. A committee spokesman did not respond to a request for comment.

Marijuana is illegal under federal law, but 20 states and the District of Columbia allow medical marijuana use. Colorado and Washington state have legalized recreational use.

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If the measure is approved, the U.S. capital would join 15 states and a handful of cities that have removed the threat of arrest for possession of small amounts of marijuana.

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Supporters have portrayed decriminalization in Washington as a way to cut law enforcement costs and increase fairness. A study by the American Civil Liberties Union has shown that eight times more black people are arrested for pot possession in the nation's capital than people of other races.