The marijuana legalization movement is in full swing this year, with 14 states currently considering ballot initiatives to legalize for either recreational or medical purposes, according to Ballotpedia. While 23 states already have legal marijuana systems in place, public opinion is quickly shifting in favor of cannabis reform (61 percent of Americans now support legalization) and voters in states across the U.S. will have the opportunity to continue the momentum in 2016.

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Legalization advocates believe that recreational marijuana, which has been adopted by four states and the District of Columbia, could make substantial gains this year. Marijuana Majority chairman Tom Angell tells ATTN: that he expects at least four states to pass recreational marijuana measures in November. He thinks this will effectively double the number of states where adults can legally purchase cannabis for personal use.

"It's going to be increasingly hard for prohibitionists, both within the [U.S. Department of Justice] and outside the government, to roll back our gains as we get more and more victories on the board," Angell told ATTN:. "Most importantly, I think it means that Congress is soon going to have to take this issue a lot more seriously than it historically has."

These states have marijuana legalization on the ballot in 2016.

1. Arizona

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There are six marijuana-related initiatives that may appear on the ballot in Arizona in 2016, including two competing measures to legalize recreational marijuana in the state.

Both recreational marijuana legalization ballots have the support of the Marijuana Policy Project, the largest organization dedicated to marijuana policy reform in the U.S.

The other ballots concern the legalization of industrial hemp, the elimination of a fee for medical marijuana applications, and a measure to legalize all drugs in Arizona.

2. Arkansas

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There are three marijuana-related initiatives that may appear on the ballot in Arkansas in 2016, including two competing measures to legalize medical marijuana and one to legalize recreational marijuana in the state.

One of the medical marijuana measures has the support of the Marijuana Policy Project, while the prospects of the remaining two remain uncertain.

Arkansas has some of the most severe marijuana laws in the U.S. — more than 5,700 people were arrested for marijuana offenses in 2012 — and the bill requires at least 100,000 signatures by July 8 to qualify for the state ballot.

3. California

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There are 11 marijuana-related initiatives that may appear on the ballot in California in 2016, including at least five measures to legalize recreational marijuana in the state.

One recreational marijuana measure, the Adult Use of Marijuana Act, is backed by multiple reform organizations and advocates, including the Drug Policy Alliance, Marijuana Policy Project, NORML, the California Medical Association, and California branch of the NAACP, and Napster founder Sean Parker. It is considered the most likely to pass.

A Probolsky Research poll conducted in February 2016 found that nearly 60 percent of California voters support the legalization of recreational marijuana, a historic high.

4. Florida

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There is one medical marijuana legalization initiative that will appear on the ballot in Florida in 2016.

The organization behind the measure, United for Care, placed a similar initiative on the state ballot in 2014, but the measure did not receive enough votes to pass.

While nearly 58 percent of Florida voters supported the 2014 initiative, the state requires a 60 percent supermajority to pass a constitutional amendment.

5. Maine

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There is one recreational marijuana legalization initiative that may appear on the ballot in Maine in 2016.

Due to a technical dispute with the notary, Maine's Secretary of State threw out some signatures in support of the initiative, but that decision could be reversed pending the outcome of litigation.

6. Massachusetts

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There are two recreational marijuana legalization initiatives that may appear on the ballot in Massachusetts in 2016.

One measure is supported by the Marijuana Policy Project and the Campaign to Regulate Marijuana Like Alcohol in Massachusetts.

7. Michigan

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There are three recreational marijuana legalization initiatives that may appear on Michigan's ballot in 2016.

A 2014 EPIC-MRA poll found that 50 percent of Michigan voters support marijuana legalization in the state.

8. Missouri

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There are two marijuana-related initiatives that could appear on the ballot in Missouri in 2016, including one measure to legalize recreational marijuana, and another to legalize medical marijuana in the state.

8. Montana

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There are three marijuana-related initiatives that may appear on the ballot in Montana in 2016, including two measures to legalize recreational marijuana in the state.

The third initiative seeks to prohibit all drugs that are currently banned under federal law. That would include cannabis, which is listed as a Schedule 1 drug in the Controlled Substances Act.

9. Nebraska

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There is one medical marijuana legalization initiative that may appear on the ballot in Nebraska in 2016.

The legalization measure is backed by both the Nebraska and Omaha branches of NORML.

10. Nevada

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There is one recreational marijuana legalization initiative that may appear on the ballot in Nevada in 2016.

The legalization measures is backed by the Marijuana Policy Project and Nevada state Sen. Richard Segerblom.

"If we do this right, this will be a major boom to tourism, which is our economy," Segerblom said of the initiative.

11. North Dakota

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There are two marijuana-related initiatives that may appear on the ballot in North Dakota in 2016, including one measures to legalize medical marijuana and another to legalize recreational marijuana.

12. Ohio

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There is one medical marijuana legalization initiative that may appear on the ballot in Ohio in 2016.

The measure is supported by the Marijuana Policy Project.

A measure to legalize recreational marijuana in Ohio failed last year due to insufficient votes. The measure was controversial even among legalization advocates, many of whom felt the initiative would create a marijuana monopoly of growing operations.

13. Oklahoma

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There is one medical marijuana legalization initiative that may appear on the ballot in Oklahoma in 2016.

The measure would reclassify marijuana as an "herbal drug" that would be regulated by the Oklahoma State Dept. of Health.

Tax revenue from medical marijuana sales would go toward education and health services.

14. Utah

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There is one medical marijuana legalization initiative that may appear on the ballot in Utah in 2016.

Two state legalization proposals were rejected by Utah lawmakers earlier this year, but advocates hope voters will push the medical marijuana measure forward.

A Dan Jones & Associates poll from November 2015 found that 35 percent of Utah voters "definitely favor" medical marijuana legalization while 26 percent "somewhat favor" the reform measure.