(Reuters) - Voters in the U.S. capital and two West Coast states will decide on Tuesday during national midterm elections whether to legalize marijuana in a test for broader cannabis legalization efforts across the United States.

Ballot measures in Oregon and Alaska would set up a network of regulated pot shops, similar to those already operating in Colorado and Washington state after twin landmark votes in 2012. A measure in the District of Columbia would allow possession but not retail sales.

The referendums come amid rapid shifts in Americans' opinions on marijuana in recent years that have seen efforts to legalize cannabis creep closer toward the mainstream and brought about sweeping pot policy changes in states and cities, where the drug remains illegal under federal law.

"Win or lose, we expect to see more support and more dialogue about the issue than ever," said Mason Tvert, a spokesman for the Marijuana Policy Project, which is working on legalization measures in California and other states for 2016.

Legalization advocates backed by national organizations have had more cash to spend in Oregon and Alaska on advertisements, get-out-the-vote drives and other campaigning, which has included a reporter quitting her job on live television and declaring her support of legal weed.

"Anything short of easy passage in all states is a major defeat for the deep pockets of the legalization advocates," said Kevin Sabet, co-founder of anti-legalization group Smart Approaches to Marijuana.

Polls have shown a narrow majority favoring legal pot, with one poll showing the opposite, in left-leaning Oregon, where voters rejected a 2012 recreational pot measure.

Polling has been inconsistent in Alaska, a Republican-leaning state with a libertarian streak. But the D.C. measure has been favored by a two-to-one margin, and advocates say it is needed to decrease the disproportionate number of blacks arrested for pot possession.

The measure would allow adults 21 and over to possess up to 2 ounces of cannabis and grow up to six plants.

Meanwhile, a proposed constitutional amendment to make Florida the 24th state and the first in the South to allow medical marijuana faces an uphill battle after well-funded conservative opposition. And two Maine cities, Lewiston and South Portland, will vote on whether to legalize the possession of small amounts of recreational marijuana.

(Reporting by Eric M. Johnson in Seattle; Editing by Nick Macfie)