The Pew Forum on Religion & Public Life has released numerous charts detailing the shifting attitudes toward marriage equality. Pew does tend to get results that are slightly more conservative than other polling firms. But what is important is the trend lines.
Pew's survey covers from 2001 until today. Had it gone back another 10 years the change would be more obviously dramatic. But in the last ten years opposition to equality of marriage rights has declined by 11 points while support has increased by ten points.
Support for marriage equality has increased generally among all age groups with one exception: those born between 1965-1980. I would, however, suggest that the 3 point drop in support there appears to be an anomaly and is fairly close to the margin of error. I can think of no reason that one age group would see support declining, even if only slightly. Older American, born 1945 or earlier have had the largest increase support. For them the numbers supporting equality increased by about 50% and now sits at one in three of all elderly Americans siding with the angels. About two-thirds of young people are supportive. The survey seems to be about gay marriage only, support in all groups would increase if civil unions were added as a "second class" option.
This only shows the level of support among the various religious groups, it doesn't indicate the shift in the sizes of these groups. Fundamentalism has been experiencing a steep decline. Many more liberal and moderate "evangelicals" have left their churches. As the more reasonable types depart these more fanatical sects the result may well be hardening of positions among the church, which becomes more and more dominated by the most rabid elements within the sect. In that sense, one could see a decline in support for marriage equality among a much smaller number of fundamentalists, meaning less opposition over all, not more.
Individuals who describe themselves as liberal strongly support marriage equality. Among moderates a majority are also now in favor of marriage equality. The hold-outs are the conservatives, as to be expected, but even there support has gone up by 8 points since 2001.
The Republicans seem to have gotten involved with a horse & buggy race but have hitched their buggy to a dying horse. The only thing that continues to save their ass is that Democrats tend to ignore the independent voters and push through measure popular with their hard-core Left support base. Independent voters are becoming increasingly disgusted by both parties. Obama's unpopular health care "reform" has a lot of independent voters returning to the GOP, not because they embrace the narrow-minded universe of fundamentalism, but because they find Obama unpalatable.
I expect the Republicans to make gains, not because independent voters think that morons like Bachmann and Santorum represent them, but simply because they are not Democrats. Obama's policies of bailout, expanded war, higher taxes, and government control of health care are not popular outside the narrow circles of the "base" with the Democratic Party. One result is that anti-gay Republicans will make some gains in the upcoming election, not because the voters are embracing the bigotry of the GOP, but because they are sick of Obama's over-reaching grabs for power. Unfortunately the rights of gay people will suffer because of Obama's thirst for power.