The older generation — those ages 65 and older — are still mostly opposed to gay marriage. In the latest survey, 42 percent of that group found it acceptable, up from 41 percent a year ago. Those in the 30-49 age category now support it by 54 percent, with those in the 50-64 category still more opposed than in favor (48 percent approved).

Young people as a group are the largest advocates, with 78 percent now in support, up from 70 percent last year.

Gallup reported that support is strongest among Easterners (67 percent), lowest in the South (48 percent), and just above 50 percent in the Midwest (53 percent) and West (58 percent).

Opinions also differ dramatically along party lines. Democrats (74 percent) are far more likely to support gay marriage as Republicans are (30 percent), while independents (58 percent) are more in line with the national average.

Though Republicans still lag behind in their support of same-sex marriage, they have nearly doubled their support for it since Gallup began polling on the question.

Said Gallup of its results: "For proponents of marriage equality, years of playing offense have finally paid off as this movement has reached a tipping point in recent years – both legally and in the court of public opinion."

This week, Pennsylvania became the 25th state to have gay marriage declared legal by voters, lawmakers or courts when a judge struck down its ban, evenly dividing the nation almost a year after a U.S. Supreme Court ruling triggered a flood of lawsuits.

Proponents of same-sex marriage have won at least a dozen consecutive victories since the Supreme Court overturned part of the 1996 Defense of Marriage Act that said the federal government could only recognize heterosexual marriages. The high court at that point also rejected an appeal of a decision that threw out a voter-approved California gay-marriage ban.

The twin decisions spurred challenges to state bans across the country. Oregon's same sex-marriage ban, having gone undefended by state officials, was struck down on May 19. This week's ruling means same-sex marriage is available to 44 percent of the U.S. population, said Charlie Joughin, a spokesman for the Human Rights Campaign, a gay rights group.

Bloomberg News contributed to this report.