WASHINGTON - US President Barack Obama's about-face in favor of same-sex marriage will galvanize both Democrats and Republicans to get out and vote, both helping and hurting the president's re-election bid, analysts said.

In a sudden reversal from his previous opposition to same-sex marriage, Obama on Wednesday became the first sitting US president to come out and support the idea, saying he has "always been adamant that gay and lesbian Americans should be treated fairly and equally."

"It is important for me to go ahead and affirm that I think same-sex couples should be able to get married," he said in a TV interview with ABC's Robin Roberts, which was aired in full on Thursday' s Good Morning America.

The pronouncement came on the heels of Vice President Joe Biden's remarks on Sunday's "Meet the Press" TV talk show that he was "absolutely comfortable" with the idea that same-sex couples and heterosexual couples are "entitled to the same exact rights, all the civil rights, all the civil liberties."

Critics blasted the administration's previous stance as confusing, as Obama had opposed gay marriage but supported same-sex civil unions. But analysts said that Obama changed course in a bid to re-assure his Democratic base.

Dan Mahaffee, an analyst with the Center for the Study of the Presidency and Congress, said the president's move will mobilize both sides of the political divide.

"The people who support gay marriage are going to be as equally motivated to vote for Obama as the people who oppose gay marriage will be to vote against him," he said.

The issue will rile up those among each party's base for whom social issues are paramount. But motivating both supporters and rivals to cast their votes could prove a double-edged sword for the president, and it remains to be seen whether his move will backfire.

Ryan Prucker, president of Imagelight/Personality Driven Media, a media consulting company, said Obama's sudden attention to the topic could leave him open to accusations of steering away from pressing economic issues, as jobs are Americans' major concerns this election season.

Mahaffee said some Republicans might even accuse the president of staging a side show to detract attention from what many believe has been his poor handling of the recovery from the worst downturn in decades.

Some analysts said Obama's change of heart could backfire in states like North Carolina, an important swing state crucial to Obama's campaign that this week became the 31st state to pass constitutional amendment to ban gay marriage.

The issue also remains controversial within the black and Latino communities, although those groups are likely to support the president's re-election bid regardless of how they feel about gay marriage.

Still, the move could help Obama's re-election campaign at a time when analysts say he will have trouble peeling his supporters off the couch to show up at the polls in November's election, as many who voted for him in 2008 have since expressed disappointment.

And many are already applauding the president for taking a stand on an issue they feel has been swept under the carpet far too long.

Meanwhile, Independents are unlikely to pay much attention to the issue, Mahaffee said, as their most crucial concerns are jobs and the economy, although polls find that more than half of Independents favor gay marriage.

Obama's sudden shift came just a day after a Gallup poll found that 50 percent of Americans support gay marriage. While Americans were once solidly against gay marriage, public opinion has moved on the issue over the past decade.

Today public opinion is split down the middle, noted John Fortier, director of the democracy project at the Bipartisan Policy Center, pointing out strong opposition to gay marriage among Republicans and strong support among Democrats.

But Fortier said he expects further increases in support for gay marriage over the next 10 or 15 years because the oldest voters are the most opposed and the youngest voters the most supportive.