Opposition to same-sex marriage dropped sharply across the country during the past two years, though just over half of Americans still oppose allowing gays and lesbians to marry, according to a poll by the Pew Research Center released Wednesday.

The poll also showed increased support for allowing same-sex couples to adopt children, and substantial backing for the rights of gays and lesbians to serve openly in the military.

The survey was released one day after a poll of California residents indicated increasing support for gay rights in the state, including for same-sex marriages. The nonpartisan Field Poll found that support for same-sex marriage in the state had risen from 38 percent in 1997 to 43 percent today.

The Pew center's national poll of 1,405 adults, conducted from March 8-12, found that 51 percent opposed same-sex marriage and 39 percent supported it. In February 2004, as same-sex couples were marrying in San Francisco, a Pew poll found 63 percent of Americans opposed the right of gays and lesbians to marry and 30 percent in favor. The margin of error in the latest survey was plus or minus 3 or 4 percentage points, depending on the question.

"In 2004, (same-sex marriage) was an emotional issue that struck a very deeply rooted chord in a lot of people," said Michael Dimock, associate director of the Pew Research Center for People and the Press. "It is still an issue -- a lot of people who opposed it then still oppose it now. But a lot of people who opposed it then were in an intense environment and either feel less strongly or feel that people can do what they want to do."

Support for same-sex marriage has grown steadily over the past decade, according to the Pew center, which is an independent research organization. In 1996, 65 percent of Americans opposed same-sex marriage and 27 percent supported it.

Wednesday's poll found the country nearly evenly split on allowing gay and lesbian couples to adopt children -- 46 percent in favor, 48 percent opposed. In 1999, 38 percent of Americans supported adoptions by same-sex couples, while 57 percent opposed them.

Sixty percent of those polled in the most recent survey supported allowing gays and lesbians to serve openly in the military, while 32 percent opposed the idea.

"It indicates people are changing," Dimock said. "They're becoming more open and tolerant, and we also have a shift in generations, which has a big impact."

The poll noted a distinct change in the number of respondents who said they "strongly oppose" same-sex marriage. In February 2004, 42 percent were in that category. That dropped to 28 percent this year, with the biggest decreases being among people over 65, Republicans and those who described themselves as religious moderates.

Gay rights advocates said Americans have had plenty of opportunity in the past two years to hear the stories of gay couples and same-sex parents, which has increased tolerance for gay and lesbian rights.

"I think people have thought more about gay families in the last two years than in the previous 30," said Matt Foreman, executive director of the National Lesbian and Gay Task Force in New York.

Any shift toward support for same-sex marriage has yet to show up at the polls, however, Since 2004, voters in 13 states have passed constitutional bans on same-sex marriage. At least seven states will vote on similar measures in November.

A representative from the evangelical Christian organization Focus on the Family declined to comment on the poll. The Family Research Council, a conservative Christian lobbying group in Washington, D.C., did not return a phone call.

©2006 San Francisco Chronicle

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