The growing acceptance and legalization of marijuana is helping the U.S. cannabis industry expand well beyond niche status. It's forecast to pump from $24 billion to $44 billion annually into the country's economy by 2020, according to an industry publication.

At the upper end of that forecast, published in the Marijuana Business Factbook 2016, it means sales of medicinal and recreational marijuana and related products and services would nearly equal the annual revenues of such Fortune 500 firms as FedEx (FDX) and Lockheed Martin (LMT), should current trends continue.

Put another way, that $44 billion is just shy of Panama's $46.2 billion GDP and ahead of Serbia's $43.9 billion. Even if the industry publication's forecast is a bit puffed up, cannabis is clearly heading for the big time.

The report said retail sales of medicinal and recreational marijuana this year are expected to grow as much as 26 percent over 2015, totaling as much as $4.3 billion. And it estimated that the overall impact of the legal marijuana industry on the U.S. economy for 2016 would be as much as $17.2 billion.

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"The projections reflect marijuana's march toward the mainstream as it emerges from the shadows to become a respectable, above-board industry that is giving birth to scores of jobs, fostering new business opportunities and creating a broad ripple effect across the country," said Chris Walsh, managing editor of Marijuana Business Daily, publisher of the report.

Growth in the industry has been sparked in part by the legalization of recreational use of marijuana in Colorado, Washington state, Oregon, Alaska and the District of Columbia. Statutes in 19 other states permit use of cannabis for medical purposes only.

But initiatives on the ballot this November could soon expand the number of states where recreational use of marijuana is legal. They include California, Arizona and Nevada -- all of which already permit the use of medical marijuana.

Last October, Gallup found 58 percent of Americans support cannabis legalization for adults, and poll results suggest that support of pot legalization will continue to grow. According to Gallup, among millennials 18 and over, nearly 3 in 4 (71 percent) said they support recreational use of marijuana.

If those millennials support legalization in even more states, or on the federal level, the industry might even blow past $44 billion by 2020.