Her chief Democratic rivals endorsed same-sex marriage as much as seven years ago, and it's widely popular with Democratic and independent voters.
By supporting gay marriage a full two years before the next presidential primary warms up, Clinton may render the issue largely settled among Democrats, should she decide to run.
But things could be vastly different in the November 2016 general election, regardless who wins the Democratic nomination. That nominee is virtually certain to support same-sex marriage, whereas there's a strong possibility the Republican nominee will not.
That could be a problem for the GOP nominee if same-sex marriage becomes a prominent issue. A poll released Monday shows a dramatic shift in attitudes about legalizing gay marriage, with 58 percent of Americans now supporting it.
Three years ago, the figure was 47 percent, the ABC News/Washington Post poll reported.
Partisan breakdowns show why it's virtually essential for a Democratic presidential hopeful to support same-sex marriage, and why it's difficult for GOP contenders to do the same.
Seventy-two percent of Democrats, 62 percent of independents and 34 percent of Republicans support same-sex marriage, the ABC-Post poll found. Unless Republicans' opinions change significantly in the next two years, a GOP presidential hopeful may struggle to win the nomination without opposing gay marriage, even if the position causes problems in the November general election.
For those who lived "through the long years of the civil rights and women's rights movements, the speed with which more and more people have come to embrace the dignity and equality of LGBT Americans has been breathtaking and inspiring," Clinton said in a six-minute video, referring to lesbian, gay, bisexual and transgender persons.
In the video, released by the gay rights advocacy group Human Rights Campaign, Clinton says gays and lesbians are "full and equal citizens and deserve the rights of citizenship."
"That includes marriage," she says, adding that she backs gay marriage both "personally and as a matter of policy and law."
Clinton recently stepped down as secretary of State, freeing her to talk more openly about U.S. domestic political matters. Some Democratic activists cautioned that her Monday statement isn't a sure sign she plans to run for the office that her husband, Bill, won two decades ago.
"I have no idea whether she is going to run or not," said veteran strategist Jim Manley. "All I know is that she was going to have to make this move quickly after stepping down as secretary of State if she was even going to think about it."
Other potential Democratic candidates got there earlier. New York Gov. Andrew Cuomo announced his support for same-sex marriage in 2006, and then made a major push in 2011 to enact it into state law. In part by promising political help to Republican legislators whose votes he needed, Cuomo claimed a major victory only two years after the same legislature had refused to legalize gay marriage.
Maryland Gov. Martin O'Malley, also eyeing a presidential bid, followed suit. After giving only partial support to a gay marriage proposal in 2011, he made it a priority in 2012, and won passage in the Democratic-dominated legislature. O'Malley, pursuing an ambitiously liberal agenda, also led the recent decision to rescind Maryland's death penalty.
Vice President Joe Biden is credited -- or blamed -- for nudging President Barack Obama to announce his support for same-sex marriage last May when Biden endorsed the idea in a televised interview.
Same-sex marriage is now legal in nine states and the District of Columbia. Civil unions are legal in eight more states, with Colorado on the verge of joining. Many other states have outlawed gay marriage.
Americans' attitudes, especially among Democrats, have tipped the balance on gay marriage in the space of one presidency. Obama, Clinton, Biden and other Democratic presidential candidates opposed legalizing same-sex marriage in 2008, although they endorsed versions of civil unions.
Clinton's bigger problem that year involved a different issue: opposition to the Iraq war, which had become deeply unpopular with Democratic voters by 2007. Clinton defended her 2002 Senate vote authorizing an invasion of Iraq. Obama, as an Illinois state legislator, had condemned the war from the start. By lagging behind Obama on this key issue as the 2008 primaries approached, Clinton lost valuable ground to the lesser-known lawmaker from Illinois.
With major Democratic politicians now taking a similar stand on gay marriage, the issue seems unlikely to play the type of role in 2016 that the Iraq war played in 2008.
President Bill Clinton signed the Defense of Marriage Act in 1996. DOMA requires the federal government to refuse to recognize same-sex marriages. It also allows states to do the same when dealing with gay or lesbian couples married in other states.
In 2000, Hillary Clinton was typical of prominent Democrats in saying marriage "has a historic, religious and moral context that goes back to the beginning of time. And I think a marriage has always been between a man and a woman."
Bill Clinton recently wrote an op-ed saying it's time to overturn DOMA. The Supreme Court will hear a challenge to the law this month.
As support for gay marriage becomes the mainstream position among Democrats, the issue is increasingly divisive among Republicans. Sen. Rob Portman of Ohio last week became the only Republican senator to support legalizing same-sex marriage. He did so after learning that one of his adult sons is gay.
Dozens of prominent Republicans have urged the Supreme Court to overturn DOMA. But many GOP-controlled states have asked the court to uphold the law.
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