As the decades-long push to legalize marijuana state-by-state nears the halfway point, Oklahoma could provide an interesting case study when voters decide whether to legalize adult cannabis use in March.

Twenty-one states have now legalized recreational marijuana use, including Missouri and Maryland, where voters this month approved ballot questions.

But voters in Arkansas, North Dakota and South Dakota, where deeply conservative politics more align with Oklahoma, recently rejected similar ballot measures.

“The low-hanging fruit for legalization has been picked, and going forward the battles could be a bit tougher,” said Chris Walsh, CEO of MJBiz, an annual cannabis business conference, which was held last week in Las Vegas.

“We are talking about (tougher battles) in deep red states … and there is going to be more organized opposition.”

Opposition to State question 820, which will come before voters on March 7, has already begun to emerge, including from Oklahoma’s Republican governor.

“It is still illegal federally. We should not have a checkerboard of jurisdiction across the states,” Gov. Kevin Stitt said last month during a debate, where he said he would oppose the ballot question.

Oklahoma Southern Baptists, the state’s largest religious group, also recently condemned SQ 820, issuing a statement that it believes marijuana has destroyed families and communities.