Nate writes that gay rights "is certainly unlikely to be pushed into the spotlight by Democrats. Most polls still show at least a plurality of Americans opposed to gay marriage, although the margin is narrowing."
I defer to Nate on questions of campaign strategy, but I would like to point out that much of this politics happens on a state-by-state level, and gay marriage does seem to have majority popularity in some states. Also, the states where gay marriage is most popular are the ones showing the largest change in the past few years. Here's the graph, from Jeff Lax and Justin Phillips:
I posted these graphs on 538 last year (also these, which show that other gay rights issues such as nondiscrimination have overwhelming popular support in most states, and these, which show attitudes by age); the gay marriage trends seemed worth reposting because of their relevance to the present discussion.