The Washington Post's Chris Cillizza analyses what he says is President Obama's calculated gamble on gay marriage.
President Obama's remarks were celebrated by Human Rights Campaign, a gay advocacy group, which said Obama had "made history" with the comments.
Neera Tanden from the Center for American Progress described the expression of support as "another large step toward realizing this country's promise of equality."
Republican gay rights activists applauded the decision but were more muted.
Christopher Barron, chief strategist of GOProud said:
– Christopher Barron, chief strategist of GOProud
"I am sure, however, the president's newly discovered support for marriage is cold comfort to the gay couples in North Carolina,"
North Carolina voted on Tuesday to join 28 other states that have voter-approved constitutional bans on same-sex marriages, according to the National Conference of State Legislatures.
US TV Networks have interrupted their programmes to announce President Obama's backing of gay marriage.
It's seen as a major political and cultural shift.
White House reports say Michelle Obama had major input, supporting the President's decision.
President Barack Obama has said he believes same-sex couples should be allowed to get married, Reuters news agency reports.
In an interview with ABC's Robin Roberts, Mr Obama said:
"It is important for me to go ahead and affirm that I think same sex couples should be able to get married.
Obama's comments mark a shift in his evolving position on the issue that is likely to please his political base and upset conservatives.
It also comes days after Vice President Joe Biden said he was comfortable with gay marriage.