In the speech marking the start of his second term, Obama placed the struggle for gay rights squarely in the pantheon of two other defining civil rights movements in American history: those for blacks and women.
"The most evident of truths -
that all of us are created equal - is the star that guides us still," he
said. "Just as it guided our forebears through Seneca Falls, and Selma, and
Stonewall." 
U.S. President Barack Obama speaks at the Commander in Chief's Ball during presidential inauguration ceremonies in Washington, January 21, 2013.
The 1848 Seneca Falls, New York, convention was an early women's rights conference. Selma, Alabama, was the site of a pivotal 1965 civil rights march demanding equality for black Americans. The Stonewall riots of 1969 were protests against a police raid of a New York gay bar and opened the door to gay rights activism.
Obama's inclusion of gay rights - still opposed by many conservatives - among his list of priorities might have been unthinkably divisive as recently as his first inauguration in 2009.
"It really speaks to how public opinion has evolved on gay rights in the last four years," said Patrick Egan, a professor of political science at New York University. "You don't see that kind of change in public opinion happen very often."
A USA Today/Gallup poll published in December found that approval of same-sex marriage had risen to 53 percent in 2012 from less than 40 percent in 2005. Young adults were the most supportive.
Nine states and the District of Columbia have legalized same-sex marriage. Last November, Maryland, Maine and Washington became the first states to do so through the ballot box.
But opposition still runs deep in parts of the country. The USA Today/Gallup poll found gay marriage opposed by a majority in the South. North Carolina in 2011 added a voter-approved ban to its constitution. Some 30 states have passed constitutional amendments banning same-sex marriage.
The issue also remains far from settled in U.S. courts. The U.S. Supreme Court in late March will hear oral arguments in a pair of cases challenging laws that define marriage as a union of a man and woman.
While many of Obama's supporters believe the president always strongly supported same-sex marriage and letting gays serve openly in the military, his public backing has only recently been on display. He was heckled in 2010 by gay rights activists who believed he was moving too slowly on policies that required gays serving in the military to be quiet about their sexual preference.
That policy, known as "Don't Ask, Don't Tell," was repealed in 2011. Vice President Joe Biden's May 2012 expression of support for gay marriage was seen at the time as getting ahead of Obama's public position.
'Made history today'
The president's speech on Monday left no doubt about his firm commitment to achieving full equality for U.S. gays.
"Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law," he said.
Other aspects of the inaugural ceremony underscored the prominence of the issue for the Obama administration. An openly gay poet, Richard Blanco, read the inaugural poem. The minister originally chosen to deliver the inaugural benediction withdrew after being criticized for making anti-gay comments.
Rights advocates welcomed what they viewed as Obama's unequivocal support.
"President Barack Obama made history today by connecting the lives of committed and loving lesbian and gay couples fighting for marriage equality to this nation's proud tradition of equal rights for all," said Chad Griffin, president of the Human Rights Campaign, a group that lobbies Congress for gay rights.
While gay rights support has traditionally been the province of Democratic politicians, many analysts see the ground shifting toward greater acceptance of gays and gay rights across the political spectrum.
While a narrow majority of Americans support gay marriage, backing for banning workplace discrimination against gays is much more overwhelming, said Egan.
"If we look at history and we look at the trajectory of support for women and African-Americans, our best guess is that politicians of the future of either party will call for equal rights for gay Americans," he said.
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