The California law allowing gay and lesbian couples to marry is likely to lead to challenges in other states where laws define marriage as only between a man and a woman.
The California law takes effect on Monday after the state's Supreme Court overturned a ban on same-sex marriages on May 15.
Same-sex marriage, one of the more divisive issues in recent U.S. politics, has mobilized millions of socially conservative Christian voters to support candidates who oppose it.
Here are the U.S. presidential candidates' positions on same-sex marriage.
DEMOCRATIC SEN. BARACK OBAMA, CAMPAIGN STATEMENT
"Barack Obama has always believed that same-sex couples should enjoy equal rights under the law, and he will continue to fight for civil unions as president. He respects the decision of the California Supreme Court, and continues to believe that states should make their own decisions when it comes to the issue of marriage."
REPUBLICAN SEN. JOHN MCCAIN
Has said he is committed to "the unique status and sanctity of marriage between a man and a woman." Voted against a proposed amendment to the U.S. Constitution which defines marriage only as the union of a man and a woman. Supports individual states' rights to regulate and determine the status of marriage within those states.
(Writing by JoAnne Allen and Paul Grant in Washington)