By Kim Palmer

CLEVELAND (Reuters) - Ohio voters will decide on Tuesday whether to become the first U.S. Midwestern state to legalize the recreational use of marijuana, although a rival ballot measure could kill the law before it takes effect.

Issue 3 would add an amendment to the state constitution that legalizes both the personal and medical use of marijuana for those over 21 years old.

If it passes, Ohio would become the fifth and most populous state to legalize the recreational usage of marijuana, following Alaska, Colorado, Washington and Oregon, as well as the District of Columbia. About two dozen states allow its use for medical reasons.

Ohio is considered a political bellwether, with the candidate who wins the state usually winning the presidency. So a victory for recreational marijuana in Ohio is expected to change the national conversation on legalization, said Gary Daniels of the Ohio American Civil Liberties Union.

Seven other states are expected to vote on recreational marijuana legalization next year, according to Danielle Keane, political director for NORML, which advocates for legalization.

Issue 3 also grants exclusive rights for commercial marijuana growth and distribution to 10 facilities across the state. Those facilities are owned by investors in the legalization movement.

Critics of the measure say that creates a monopoly, and responded with a rival ballot measure called Issue 2. That ballot measure would nullify legalization if it creates "an economic monopoly or special privilege" for a private entity.

NORML has endorsed the ballot measure, although with "some hesitancy" because of the limited number of growing sites, Keane said.

Ohio State University constitutional law professor Daniel Tokaji said he believed the legalization measure would fail to pass because of the word "monopoly" in the ballot language.

Recent polls were split down the middle for legalizing recreational use in Ohio.

Ian James, executive director of Responsible Ohio, a political action group that brought the issue to the ballot, said volunteers had knocked on a million doors in the weeks leading up to the election in part to educate voters to vote "no" on Issue 2.

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James acknowledged the vote would be close. Mild weather is expected on Tuesday in Ohio, which could help turnout.





(Reporting by Kim Palmer; Editing by Mary Wisniewski and Peter Cooney)