WASHINGTON — In the late 1980s, support for gay marriage was essentially unheard of in America. Just a quarter century later, it’s now favored by a clear majority of Americans.

That dramatic shift in opinion is among the fastest changes ever measured by the General Social Survey, a comprehensive and widely respected survey that has measured trends on a huge array of American attitudes for more than four decades.

Support for the right of same-sex couples to marry has risen 8 percentage points in the past two years and jumped 45 points since the question was first asked in 1988, when only 11 percent of Americans said they agreed with the idea. The survey now finds that only one-third of Americans are opposed to gay marriage.

The largest shift in support since 2012 has come among Republicans, just under half of whom – 45 percent – now support marriage rights for same-sex couples. That’s a jump of 14 percentage points since 2012.

“Many things don’t change a lot. Most things change very slowly,” said Tom W. Smith, director of the General Social Survey. “This is one of the most impressive changes we’ve measured.”

The General Social Survey is conducted by NORC, an independent research organization based at the University of Chicago, with funding from the National Science Foundation. It is a highly regarded source of data about social trends because of its long-running and comprehensive set of questions about the demographics and attitudes of the American public.

Data from the 2014 survey was released this week, and an analysis of its findings on gay marriage was conducted by The Associated Press-NORC Center for Public Affairs Research and the General Social Survey.