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 US politics, has mobilized millions of socially conservative Christian voters to support candidates who oppose it.
Here are the US presidential candidates' positions on same-sex marriage.
Democratic Senator Barack Obama's 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 Senator 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 US 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.
