Barack Obama has been both praised and criticised a day after he became the first sitting US president to publicly support gay marriage.
Social conservatives and religious leaders condemned his remarks.
Meanwhile, the Obama campaign attacked Republican Mitt Romney, who restated his opposition to same-sex marriage, as out of touch on the issue.
Mr Obama travelled to the West Coast on Thursday for fundraisers in Seattle and Los Angeles likely to raise millions.
One fundraiser, to be held at the home of George Clooney, is expected on its own to raise $15m (£9.3m), partially from a general raffle offering members of the public the chance to meet the Hollywood actor.
In the wake of his interview with ABC News, gay advocates applauded Mr Obama's remarks.
Human Rights Campaign President Joe Solmonese said that the president's comments would "inspire thousands more conversations around kitchen tables and in church pews".
But Cardinal Timothy Dolan, president of the US Conference of Catholic Bishops, labelled Mr Obama's remarks "deeply saddening".
"We cannot be silent in the face of words or actions that would undermine the institution of marriage, the very cornerstone of our society," he said in a statement. "The people of this country, especially our children, deserve better."
On Thursday morning, the Obama campaign sought to capitalise on the president's political gamble by releasing an internet video titled Mitt Romney: Backwards on Equality .
It shows a clip of Mr Romney, the Republican who is expected to challenge Mr Obama for the White House in November, saying on Wednesday that he opposes gay marriage.
The video says that even former Republican President George W Bush supported civil unions, a step short of marriage.
On Wednesday evening, the Republican-controlled House of Representatives moved to reinforce the Clinton-era Defense of Marriage Act (Doma).
By 245-171, lawmakers voted to prevent the justice department from using taxpayer funds to actively oppose the act, which prevents gay marriages from being recognised at the federal level.
Mr Obama ordered the department to stop actively defending Doma in February 2011.
The vote's sponsor, Kansas Republican Tim Huelskamp, said it was not Mr Obama's "prerogative" to decide "which laws matter and which do not".
Mr Obama broke from his long-claimed indecision on the issue of gay marriage to express outright support for the right of homosexual couples to marry, in an interview on Wednesday with ABC News.
The US president acknowledged that his interview had been prompted by Vice-President Joe Biden's remarks on Sunday that he was "absolutely comfortable" with gay marriage.
He said he had planned to speak on the issue before the Democratic convention in September, and would have preferred to have "done this in my own way, on my own terms without, I think, there being a lot of notice".
In 2010, Mr Obama said his views on the issue of gay marriage were "evolving", a stance that had frustrated gay rights supporters and donors.
However, Mr Obama's newly declared stance does little to change the legal status for gay people who wish to wed in states where such marriages are outlawed. Thirty-one US states have passed constitutional amendments or legislation against same-sex marriage.
'Bible's against that'
On Tuesday, North Carolina approved a constitutional amendment - 61% in favour and 39% against - effectively banning same-sex marriage or civil unions.
A Gallup poll on Tuesday suggested that 50% of Americans were in favour of legalising gay marriage - a slightly lower proportion than last year - while 48% said they would oppose such a move.
Mr Obama's announcement is seen as politically risky in the upcoming election, especially in the South, where one in three swing voters strongly opposes allowing gays and lesbians to wed. Mr Obama narrowly won North Carolina in the 2008 election.
BBC North America editor Mark Mardell says the Obama campaign hopes the announcement will energise younger voters.
But Mr Obama's remarks may not play so well with religious African-American voters, a key Obama voting bloc. Recent polling suggests that support for gay marriage among black church-goers remains lower than many other groups.
Pentecostal Pastor Charles Bargaineer, of the largely black New Fellowship Church of God in Florida, told the Associated Press he was troubled by the president's position.
"I don't think that's appropriate for the president," Mr Bargaineer told Reuters news agency. "The Bible's strictly against that."
When asked whether he would vote again for Mr Obama, Mr Bargaineer said: "I'll have to pray about that."
Reverend Scott Clark, a gay pastor from the San Francisco Theological Seminary, said it had been "deeply moving" to hear Mr Obama "finally acknowledge the full dignity and humanity of lesbian, gay, bisexual, and transgender people and our families".