Support for marijuana legalization is up strongly over time. Pew finds that support and opposition is nearly evenly split (45% in favor; 46% opposed). This represents a long term trend toward legalization.
Will support continue over time? The answer is likely yes. “Young people under the age of 30 favor legalizing the use of marijuana by a 54%-42% margin. Opinion is divided among those in middle age groups. Those 65 and older are broadly opposed to legalization (66% illegal, 30% legal).”
If generational replacement leads to the replacement of the most ardent opponents in the electorate with more supporters, legalization will become the majority position.
Of course, what will be most interesting to see at what point a mainstream politicians will support legalization, since there is nearly universal opposition to legalization among politicians.