That public attitudes have changed throughout the years regarding marijuana can easily be observed by viewing the 1936 propaganda film Reefer Madness, originally financed by a church group under the title Tell Your Children, According to this film, marijuana was so dangerous that it led people to be so "drug-crazed" that they committed murder, attempted rape, and descended into sheer madness. Later, in the 1970's the film was redistributed as satire by those who advocated cannabis policy reform.
One of the major reasons for the popular opinion about legalized marijuana having changed is the change in social perceptions and reactions. In 1936 when Reefer Madness was made the "drug revolution" of the 1960s, of course, had not occurred; consequently, drug use was not prevalent among the middle class and upper classes. Therefore, perceptions were that only the criminal or "lower" element partook of such things. The conventional wisdom of the 1930's was that it was a drug used mainly by Hispanics in the Southwest, an ethnic group that was certainly not favored in the United States. According to David F. Musto, author of The American Disease: Origins of Narcotic Control, by depicting this ethic group as having an illegal pastime was a method of stigmatizing Hispanics and rallying the support of the public to remove this population to Mexico.
Added to the counterculture of the 1960s and its attitudes to drugs, new scientific discoveries about the effects of cannabis--notwithstanding a popular American president's having admitted to smoking marijuana in his youth (even though he "did not inhale") as well as the current president admittedly having smoked marijuana, have effected changes in attitudes about this drug, especially when it is put up against such drugs as LSD, cocaine, and heroin that are used by those in all levels of society.
In the last decade or so, medical studies have verified the medicinal benefits to some that marijuana has. California has long had marijuana legalized for those suffering from glaucoma and other medical conditions. Furthermore, in arguably the most conservative of states, Alabama, marijuana is now available in this medical respect.
Also, there has been a push since the 1990's to remove the illegality of marijuana as scientific studies have shown it to rarely lead to violence as alcohol often does; in addition, it is argued that marijuana is no more dangerous. Certainly, popular opinion has changed. In 2002 a survey revealed that 61% wanted marijuana to be illegal while 32% wanted it legalized; in 2013, 50% illegal 45% legal.
Another influence in changing attitudes toward marijuana is the practicality of legalizing it. As one Federal Correction Institution inmate privately told an Educational Specialist [known to this educator] employed there, "The worst thing that could happen to us (cannabis dealers) is for the government to legalize it. This would put us all out of business." Legalizing marijuana would reduce prison populations with the minor players and where there are "mules" who unwittingly loaded crates of marijuana onto planes, etc. simply because they were peasants who needed money. It is argued that these people did not even know that they were committing crimes and should not be imprisoned and wasting taxpayers' money.
Certainly, since the legal sale of marijuana began in Colorado, it is apparent that this state has benefited in tourist trade, taxes, and increased business income. With some states that have $millions+ in unfunded mandates, the legalization of marijuana could possibly help relieve some financial problems, so citizens are of the opinion that legalized marijuana would have a positive effect.
In an article by Jason Eastman, he argues that the sociological understanding of what is identified as "deviance" determines public opinion, and he notes that a recent survey by PEW reveals that "our perceptions of this drug (and therefore its definitions of use as deviant)" have clearly been altered. Therefore, it is likely that federal laws regarding what is no longer viewed as a "deviant" will thus be changed.
For a long time pro-marajuana use was an underground counter culture movement. It has since moved into the spotlight and grown significantly.
I believe the main cause of this has to do with sponsors. The stereotypical pot head is no longer the poster boy for marajuana use. Cancer survivors, business men and even moms have come foward in support of legalizing marajuana. The public at large can identify with these backers. You no longer have to be a hippy or a rock star to support legalization. These changes in image assoaciation have led to rising support of marajuana legalization.