Ohio voters will decide Tuesday on whether to become the first Midwestern state to legalize the recreational use of marijuana, though 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 personal and medical use of marijuana for those over 21 years old.

The ballot initiative was the result of a campaign that gathered more than 300,000 valid voter signatures from around the state.

If it passes, Ohio would become the fifth, and largest, state to legalize the recreational usage of marijuana, following Alaska, Colorado, Washington and Oregon, as well as the District of Columbia.

Ohio is considered a political bellwether - the candidate who wins Ohio usually wins the presidency. So a win for recreational marijuana in Ohio is expected to change the national conversation on legalization, according to 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.

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

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

"We support marijuana legalization, but we cannot support Issue 3," said Maurice Thompson, executive director of 1851 Center for Constitutional Law, a conservative legal rights organization. The Ohio Green Party also opposes Issue 3 over the monopoly issue.

Ohio State University constitutional law professor Daniel Tokaji believes that marijuana legalization measure will fail to pass due to the word "monopoly" in the ballot language.

But Thompson's group and the ACLU also are concerned that the anti-monopoly measure could tie up other citizen-initiated amendments.

If both measures pass, the conflict will likely end up in court, said Daniels.

Recent polls in the state are split down the middle for legalizing recreational use - support is greater for medical use.

Ian James, executive director of ResponsibleOhio, a political action group which brought the issue to the ballot, said that the measure is not about monopolies but "providing access to adults and smothering a black market."

ResponsibleOhio volunteers have knocked on a million doors in the weeks leading up to the election in part to educate voters to vote "no" on Issue 2, James said.

"Ultimately it is going to be all about the turnout," said James.

(Reporting by Kim Palmer; Editing by Mary Wisniewski and Alan Crosby)