Here is an ever-developing list of the various positions of the 2012 presidential candidates on marijuana:

Ron Paul

Ron Paul is well-known for his libertarian approach to issues, and marijuana and the War on Drugs is no different. Dr. Paul believes that

Mitt Romney

Mitt has stated clearly his opposition to recreational and medical marijuana. Here's a clip of Mitt Romney scoffing at a medical marijuana patient:

And here is Ron Paul graciously answering that same medical marijuana patient's questions:

Rick Perry 

Rick Perry

In his book, Fed Up, Perry says that medical marijuana is an issue that should be left up to the states. In an interview on the subject of drugs, a Perry spokesman stated that "The Governor does not support legalizing any drug. The Governor supports federal drug laws where appropriate. And while the Governor is personally opposed to legalizing the use of medical marijuana, if states want to allow doctor prescribed medical marijuana, it seems to him that under the 10th amendment, they have the right to do so."

From Rick Perry's book, Fed Up:

“Crucial to understanding federalism in modern-day America is the concept of mobility, or "the ability to vote with your feet." If you don't support the death penalty and citizens packing a pistol, don't come to Texas. If you don't like medicinal marijuana and gay marriage, don't move to California....

The Court subsequently ruled that Congress, using its Commerce Clause power, could prevent California from legalizing medicinal marijuana. In other words, the federal government has the full prerogative to intervene in your private home if you are engaged in any activity that has some minimal relationship to the exchange of goods....

When the federal government oversteps its authority, states should tell Washington they will not be complicit in enforcing laws with which they do not agree. Again, the best example is an issue I don't even agree with—the partial legalization of marijuana. Californians clearly want some level of legalized marijuana, be it for medicinal use or otherwise. The federal government is telling them they cannot. But states are not bound to enforce federal law, and the federal government cannot commandeer state resources and require them to enforce it.” 

Newt Gingrich   

Newt Gingrich is an infamous drug warrior who has been steadfast in his opposition to marijuana legalization. He infamously co-sponsored the Drug Importer Death Penalty Act of 1996, under which importing more than two ounces of certain illegal substances into the country could be punishable by life imprisonment or death.

Here a self-identified recreational drug user asks Newt Gingrich if she should be arrested:

The Other Republican Candidates

Gary Johnson

The other libertarian-leaning presidential candidate other than Ron Paul also has predictably similar views on marijuana and drugs. Johnson advocates complete legalization of marijuana as well as a harm-reduction based approach to all other drugs. He has even gone as far as suggesting he would pardon all nonviolent marijuana offenders, making Johnson perhaps even more pro-pot than Ron Paul.  

Johnson also openly admits previously using marijuana for medicinal reasons, including using it after a paragliding accident in 2005.

Finally, Johnson believes tjay"Pot smokers may be the largest untapped voting bloc in the country," he said in an interview with Outside Magazine. "A hundred million Americans have smoked marijuana. You think they want to be considered criminals?" Good point, Gary.

Jon Huntsman

 Jon Huntsman, like Ron Paul, believes legalization is a state issue and should be left up to the states to decide.

Here's a video showing Jon Huntsman's support of medical marijuana:

Rick Santorum 

 Rick Santorum, well known for his social conservative crusading, is an avowed drug warrior, seeing marijuana and other drugs as harmful to the family.

Despite his opposition, Santorum reluctantly admits to using marijuana in college, stating on Piers Morgan Tonight that “Well, yeah, I admitted you know, back when I was running for the Senate, that when I was in college that I smoked pot and that was something that I did when I was in college. It was something that I’m not proud of, but I did. And said it was something that I wish I hadn’t done. But I did and I admitted it. I would encourage people not to do so. It was not all it’s made up to be.”

 Here Rick Santorum openly admits his ignorance towards medical marijuana

Rick Santorum also claims that the federal government doesn't arrest non-violent drug offenders

Herman Cain 

After initially dodged the question at the 2011 Ames Straw Poll (not unusual for Cain, Herman Cain finally clarified his position on medical marijuana in an interview in Iowa with MSNBC. Cain now claims that medical marijuana is an issue of states rights and that states that choose to legalize medical marijuana will not be subject to federal interference in a Herman Cain presidency.

Michele Bachmann

 Michele Bachmann does not have a publicly-known record on marijuana or drugs. However, the social conservative congresswoman did vote against an amendment to a 2007 bill, H.R. 3093, which sought to prevent states from using federal funds made available under the law to be used in the distribution, possession, or cultivation of medical marijuana.

 So there you have it. A complete list of all of the positions of the 2012 candidates on marijuana.