This is a perfect issue for several reasons. First, there is a large liberal base of people who believe medical pot should be legal. Second, several red states have already made it legal. Alaska, Montana, Arizona, Colorado to name some.
Also, it can be framed as a states rights issue, just like gay marriage. Why should the feds arrest people who are in a state that approves medical marijuana, not to mention, why should they arrest people following doctors advice?
It is an issue that can be used with controversial and effective advertising. Find a cancer patient and have them ask why the goverment wants to take their medicine.
I'm sure the democrats are too spineless to touch something with the word 'marijuana' in it though.