While November 8 could be a watershed moment for marijuana, there's some big money fighting legalization. REUTERS/Mark Blinch

Election Day could be a watershed moment for marijuana.

Five states, including Arizona, California, Nevada, Maine, and Massachusetts, will be voting to legalize recreational marijuana on November 8.

According to the latest poll, Americans overwhelmingly favor legalization.

However, there's still some serious opposition to legal marijuana. And the funding is coming from a number of pharmaceutical companies, prison suppliers, and casino magnates.

"It's obvious that a growing majority of Americans support legalization, and the only way they have a hope of maintaining prohibition is to put up a bunch of Reefer Madness-inspired ads in an attempt to scare voters, which costs a lot of money," Tom Angell of the pro-legalization group Marijuana Majority told Business Insider. "But it won't work."

Insys Therapeutics, an Arizona-based pharmaceutical company, gave $500,000 to Arizonans for Responsible Drug Policy, one of the most prominent anti-marijuana groups in the state.

Insys recently developed Dronabinol, a synthetic cannabinoid compound intended to reduce the nausea associated with chemotherapy. While Dronabinol was recently approved by the FDA, Insys said in a recent SEC filing that marijuana legalization could "significantly limit the commercial success of any dronabinol product."

"If marijuana or non-synthetic cannabinoids were legalized in the United States, the market for dronabinol product sales would likely be significantly reduced and our ability to generate revenue and our business prospects would be materially adversely affected," the filing said.

Insys isn't alone, either.

Angell points to companies like prison suppliers, which cater meals to correctional facilities, as potentially having a lot to gain from keeping marijuana illegal. Marijuana offenses contribute considerably to the number of prisoners in correctional facilities.

Even in California, which has relaxed marijuana laws, thousands of people are still incarcerated for low-level marijuana offenses, according to a report from the Drug Policy Alliance (DPA) based on data analyzed by New Frontier, a market-research firm. The report was provided to Business Insider and published on New Frontier's website.

In 2015, an estimated 2,139 people were jailed for marijuana offenses in California and another 4,389 people were jailed for "Marijuana Plus" offenses — where low-level marijuana crimes were included in the sentence. That's a total of 6,537 jailed for marijuana in 2015, according to the DPA.

Those numbers are actually a decline since 2011 — when possession of personal amounts of marijuana was reduced by California from a misdemeanor to an infraction.

"Accepting these contributions from companies with such naked self-interest shows just how desperate our opposition is becoming," Angell said.

While California is likely to pass marijuana legalization, polls show a much tigher split in Arizona.

In Arizona, posession of a personal amount of marijuana — defined as less than two pounds — is a felony charge subject to four months to two years of jail time.

Meanwhile, Las Vegas casino magnate and prominent Donald Trump donor Sheldon Adelson has kicked $2 million of his own money to the anti-legalization effort in Nevada, reports The Washington Post's Christopher Ingraham.

Adelson's opposition to marijuana may be because of his business interests, according to Mason Tvert, the director of communications for the pro-legalization group Marijuana Policy Project.

"If you like drinking alcohol and playing blackjack at the casino, Mr. Adelson wants you to be his guest," Tvert told Ingraham. "If you prefer to consume marijuana while playing video games in the privacy of your home, Mr. Adelson wants you to be in jail."

Adelson has also contributed $500,000 to fight marijuana legalization in Arizona.

In Massachusetts, the alcohol industry has chipped in to defeat legalization measures. The Beer Distributors of Massachusetts and the Wine & Spirits Wholesalers of Massachusetts, two trade associations, have contributed $75,000 to the Campaign for a Safe and Healthy Massachusetts, an anti-legalization group, reports The Boston Globe.

Alcohol vendors likely fear that marijuana will hurt their business, but there's scant evidence to support that claim. The alcohol industry in Colorado has actually grown since marijuana was legalized for recreational use, according to 2015 tax records.

Some opposition to marijuana legalization in Massachusetts is coming from healthcare providers as well.

"Proponents of legalizing recreational marijuana use argue that it will save lives by giving people an alternative to opioids for pain relief," Kurt Isaacson, the CEO of Spectrum Health Systems, a for-profit addiction clinic with locations in New England wrote in Stat News. "What they don’t consider is the detrimental impact of recreational marijuana on youths."

This could a be a lot more legal after November 8. Istock

The anti-legalization fight in Maine is being led by Mainers Protecting Our Youth and Communities, a political action committee with funding from a diverse group of law enforcement unions, the Roman Catholic diocese in Portland, and the office of the Governor Paul Lepage.

Maine's pro-legalization PAC, Campaign to Regulate Marijuana Like Alcohol, has vastly outspent anti-legalization efforts however, reports The Portland Press Herald. The Campaign to Regulate Marijuana like Alcohol raised $904,145, according to a finance report filed to the state ethics commission, compared to $50,000 over the same time period for the anti-legalization side, reports The Press Herald.

Voters in Maine are favorable to legalizing marijuana, with recent polls showing between 50 and 55% support for legalization.

Editor's note: This story has been updated based on information provided by Services Group America.