Declining tobacco use could soon intersect with a steady uptick in the smoking and consumption of another leafy drug – marijuana.

Marijuana use among American adults has seen such a large spike after partial legalization that it could overtake the number of tobacco users within several years. Reported marijuana use has nearly doubled in the last three years: from 7 percent of Americans in 2013 to 13 percent today, according to a Gallup poll released Monday.

This change puts the number of regular marijuana users in the United States at 33 million, compared to 40 million tobacco users, a number that is dropping after decades of government and community campaigns against it. If use of both drugs continues along the current trend line, marijuana use will surpass that of tobacco in several years, as The Washington Post notes.

Decriminalizing pot has become increasingly popular among Americans in recent years, as Gallup found 58 percent of Americans supported legalization in 2015, but pollsters note that legalization in some states could have a causative effect on marijuana use as well.

During the three-year period when marijuana use doubled, so did the number of states that decriminalized recreational marijuana. Just Colorado and Washington had legalized the drug in 2013, but they have since been joined by Alaska, Oregon, and the District of Columbia, and several others have at least partially decriminalized it. Five other states will vote on whether to legalize in November, although federal law remains steadfastly against it.

"States' willingness to legalize marijuana could be a reason for the uptick in the percentage of Americans who say they smoke marijuana, regardless of whether it is legal in their particular state," Justin McCarthy wrote for the poll service. "Gallup finds residents in the West – home of all four states that have legalized recreational marijuana use – are significantly more likely to say they smoke marijuana than those in other parts of the country."

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Although marijuana legalization is becoming increasingly popular, some Eastern states, however liberal in their politics, seem more conservative in their approach to the drug than those in the West, as The Christian Science Monitor noted in May:

Political leaders in liberal Massachusetts, the state that introduced an Obamacare-like system, balk at legalizing marijuana. And Vermont, home of socialist Sen. Bernie Sanders, criticized Massachusetts's marijuana initiative as too lax and killed its own fledgling pot bill. 'We in the Commonwealth would be better watching and learning from the case study of Colorado for five or six years, rather than just two,' [Republican Massachusetts state Sen. Viriato deMacedo, one of nine senators who went on a four-day fact-finding mission to Colorado] tells The Christian Science Monitor in an interview.

Although many of its government officials profess opposition to legalizing the drug, Massachusetts is one of the five states that will vote on recreational use via referendum in November, along with California, Arizona, Nevada, and Maine.

While regular marijuana use has increased, the share of Americans who have "ever happened to try marijuana," as the poll asks, has also risen at a fairly steady rate. It was concentrated among the young beginning in the 1960s and has gradually increased as that generation ages, from 11 percent of respondents in 1972 to 43 percent today.