My estimate is 16 percentage points. Nate Silver estimates perhaps two or three percentage points a year and, according to a leaked memo, Republican pollster Jan van Lohuizen finds support rising one point a year until 2009 and 5 points a year since.But, support varies dramatically from state to state.
Opposition to SSM varies widely by state. Seong Soo Oh and I [Gregory P. Lewis] concluded that support was 30 points higher in Massachusetts than in Mississippi in 2006. Jeffrey Lax and Justin Phillips found a 40 point split between Massachusetts and Utah in 2009. My most current estimates find nearly a 50 point division between Massachusetts and Mississippi.As of 2011, Colorado was third in the nation in the percentage of the voters supporting same sex marriage (behind Rhode Island, which also does not have same sex marriage, and Massachusetts, which does have it). In part, this is due to the "present company excluded" norm of interpersonal etiquette. Denver has sent at least three openly gay legislators to the Colorado General Assembly in recent years: Jennifer Veiga, Mark Ferrandino and Pat Steadman (I've been represented at one point or another by all three), and we have an openly gay member of Congress, Jared Polis (CO-2 which includes Boulder, Colorado). It has hard for legislators to vote in favor of discrimination against the colleagues and that trickles down to the entire political elite in the state. Major political and civic contributions from affluent gay Coloradans like Tim Gill, have also had an impact.
Despite the defeat of the Civil Unions bill this year in Colorado, every Democrat in the Colorado General Assembly, at least five state house Republicans, and several state senate Republicans supported the bill. The percentage of legislators willing to vote for the bill is within the margin of error of the percentage of voters who support it. Its defeat this session was a product of the imperfections of parliamentary procedure in translating the will of individual legislators into enacted legislation.
Interestingly, and supporting the trend of same sex marriage support as a regional issue, as much as a partisan issue, there is majority support for same sex marriage in the Republican stronghold of Alaska. In general, same sex marriage opposition seems to track Evangelical Christianity (including Mormons) more than partisanship.