When Gallup first asked Americans how they felt about marijuana in 1969, only 12 percent of respondents favored the legalization of weed. That number has increased steadily with each passing decade, and in October 2011, Gallup reported that 50 percent of Americans favor the legalization of marijuana, the country's most popular illicit drug.

The shift in popular opinion reflects not just decades of scientific research showing that marijuana is relatively safer than both alcohol and harder drugs (including many prescription pills), but also the savvy PR efforts of drug reform wonks and activists. When even conservative Christians such as The 700 Club's Pat Robertson are calling for legalizing pot, you know that the war on the War on Drugs is not just winnable, but practically over.

But that doesn't mean all arguments in favor of legalization are equally good, effective, or factual. Here are the three weakest arguments for legalizing marijuana. As you work to convince the shrinking ranks of drug prohibitionists—we're looking at you, Mr. President!—don't make these rookie mistakes when arguing for changing the legal status of cannabis.

3. Legalizing Marijuana Will End Cartel Violence in Northern Mexico

The election of Mexican President Felipe Calderon in 2006 ushered in a new era of prohibition-fueled drug violence. Six years and 50,000 drug-war deaths later, the argument that repealing marijuana prohibition could stem the violence in Mexico and along the U.S. border is ubiquitous. The claim was a major selling point for Proposition 19 in California, which would have legalized marijuana and subjected its sale to taxation and regulation, and has been made repeatedly by drug reform advocates in the two years since.

"We have created an illegal marketplace with such mind-boggling profits that no enforcement measures will ever overcome the motivation, resources and determination of the cartels," Libertarian Party presidential candidate Gary Johnson wrote in a 2011 op-ed for The Washington Times. Legalizing pot, he added, would deny the cartels "their largest profit center and dramatically reduce not only the role of the United States in their business plans, but also the motivation for waging war along our southern border."

But there are objections to that claim. In October 2010, the RAND Corporation released a study saying that Mexican cartels derived only 16 percent of their revenue from marijuana. (As pointed out by NORML, that number conflicted with the ONDCP's estimate that 61 percent of cartel revenue comes from marijuana.)

In June 2011, Mexico analyst Sylvia Longmire argued that cartels have diversified to the point that legalizing marijuana might dent their war chests, but it won't stop them; they'd still make money stealing oil from pipelines, pirating and selling contraband intellectual property, extorting small businesses, bribing politicians, ransoming kidnap victims, manufacturing and moving harder drugs such as cocaine, heroin, and meth, and trafficking undocumented immigrants and sex workers.

In 2011, David Borden, executive director of StoptheDrugWar.org, emailed me with objections to Longmire's argument: "Some of the other criminal enterprises that cartels are involved in (enterprises they've been able to enter because of having drug cash and organizations built by drug cash) are less straightforwardly tied to demand, such as kidnapping for ransom, but they have their limits—for all we know they are already doing as much of those things as they think could be sustained, and the more profit they continue to make from drugs, the more money they are going to invest in all kinds of enterprises, both illicit and licit."

"Will the cartels vanish from the face of the earth because of marijuana legalization?" Borden continued. "Probably not. Would even full legalization of all drugs accomplish that? Unclear."

That lack of clarity is exactly why marijuana reformers should be careful when promising what legalizing pot can and can't do for Mexico. The war on drugs has weakened the country's political institutions, corrupted its military and police forces, and devastated its economy. While pot legalization in the U.S. would allow users to divest from the cartels' brutality, pitching marijuana legalization as anything other than a baby step toward peace and stability in Mexico puts drug reformers on tenuous grounds.

NEXT: Marijuana should be taxed and regulated because it is America's largest cash crop.

2. Marijuana Should Be Taxed and Regulated Because It Is America's Largest Cash Crop

In 2006, ABC News reported that with "a value of $35.8 billion, marijuana exceeds the combined value of corn ($23.3 billion) and wheat ($7.5 billion)." That number came from a report published by Jon Gettman, director of the Coalition for Rescheduling Cannabis. Gettman arrived at this figure by multiplying the estimated number of metric tons of marijuana cultivated in the U.S. in 2005 (10,000 tons, or 20 million pounds) by a production value of $1,600 per pound.

Drug law reformers claimed Gettman's report was evidence that eradication and enforcement efforts had failed. In the intervening years, however, the statistic has been used to make the case that taxing and regulating marijuana would solve many of America's fiscal woes. The former argument is a sound one, the latter is not.

Here's why: Gettman's estimate of $1,600 per pound was conservative when compared to law enforcement agencies, which in 2005 cited the street value of marijuana at between $2,000 and $4,000 a pound. Marijuana cultivated in a post-prohibition market, however, would cost a fraction of that.

"To get a sense of the disparity in price between legal and illegal drugs," Reason's Jacob Sullum wrote in 2007, "compare the production value of marijuana—about $1,600 per pound, by Gettman's estimate—to the production value of tobacco, a legal psychoactive weed that U.S. farmers sell for less than $2 per pound."

Let's go back to 2005, make marijuana legal, and give it an astronomically high production value of $800 per pound, or half of Gettman's black market estimate: It would have tied with soy beans in 2006 as America's third largest cash crop, with an average production value of roughly $17 billion. If it had the same production value per pound as tobacco, or $2, its APV in 2005 would have been $44 million; or less than 10 percent of beans, 2005's 20th most valuable cash crop.

So while pointing to marijuana as America's largest cash crop is a good indicator of its popularity (and arguably, the safety of its use), it doesn't follow that taxation and regulation of the drug in a post-prohibition market would be an unlimited boon to government coffers, especially when factoring in the costs of an aggressive regulatory framework.

NEXT: Marijuana should be legal because it's medicine.

1.) Marijuana Should Be Legal Because It's Medicine

There's no question that marijuana eases pain, stimulates the appetite, reduces nausea, and helps with a slew of other physical and psychological ailments. There is some question, however, as to whether promoting it as medicine is the best political strategy for making it fully available as a recreational drug.

Earlier this year, NORML Executive Director Allen St. Pierre wrote a searing critique of the medical marijuana strategy.

"If this were the 1920s, advocacy of today's 'medical' cannabis industry would sound like a lawyer back then fronting for the legal sellers of 'prescription' alcohol during Prohibition. Prescriptive alcohol was a sham then, and the 'medical' cannabis industry (not medical cannabis itself) is largely a sham now."

"Cannabis consumers," he continued, "who NORML represents, want good, affordable cannabis products without having to go through the insult and expense of 'qualifying' as a 'medical' patient by paying physicians and/or the state for some kind of get-out-of-jail-free card. How intellectually honest is all of this?"

One response is that successful medical marijuana ballot initiatives protect people who use marijuana for genuine medical reasons from harassment and imprisonment. But the problems with those laws–such as who counts as a caregiver, and the number of prescriptions given to people who are using it recreationally–don't reflect well on the political acumen of drug law reformers.

Legislators and regulators are wising up and changing tactics. Because most states that currently have medical marijuana laws make the bulk of their sales to people with chronic pain—the only ailment eligible for medical marijuana that doctors can't test for, and thus the ailment most likely to be cited by recreational users looking for safe access—Washington, D.C. decided to omit chronic pain from its list of ailments that qualify for medical marijuana. In the District, only people with cancer or a terminal illness will be able to get medical pot. In Colorado, where legislators claim only 20 percent of marijuana sales are to people with "legitimate" illnesses such as HIV/AIDS, cancer, Crohn's disease, and MS, legislators are looking for ways to limit the number of recommendations doctors can write to the other 80 percent of users.

In short, while medical marijuana laws initially gave more users safe access, anti-pot legislators now seem to know that the best way to limit marijuana sales is to treat it exactly like advocates claim to want: as medicine subject to a strict and invasive regulatory prescription scheme.

Mike Riggs is an associate editor at Reason magazine. Follow him on Twitter.

Read Reason's drug policy coverage here.

