The important finding to take away here is that while some polling has a majority in favor of marijuana legalization and some doesn’t, there isn’t much change in the numbers found by each individual pollster; what variation there is tends to be what one would expect from sampling error.
For me, the most important poll on the subject of marijuana legalization is the General Social Survey. It’s not a poll that most in the public have heard about, but it’s well known within academic and research circles. Conducted by the University of Chicago every couple of years, the poll has a near 70% response rate compared to only about 10% for most public surveys. It ensures that tougher-to-survey demographics, like African Americans and Latinos, are better polled.
The 2012 General Social Survey (pdf) demonstrates, like most other polling, that support for weed legalization is up over the past 25 years, yet shows no clear trend over the immediate past. In fact, the percentage who wanted weed to be made legal actually dropped from 44% in 2010 to 43% in 2012. And 43% is, of course, not a majority.