BOSTON (AP) - A timeline of events in the campaign to legalize same-sex marriage in the United States:

- 1993: The Hawaii Supreme Court rules that denying marriage to gay couples violates the state constitution, sparking a push in Congress to pass pre-emptive federal legislation.

- 1996: President Bill Clinton signs the Defense of Marriage Act, which defines marriage as a union between one man and one woman and denies gay couples a host of federal marriage benefits.

- 1998: Hawaii amends its constitution to give the Legislature sole power over amendments regarding gay marriage. The same day, Alaska voters amend the constitution to limit marriage to heterosexual couples.

- 1999: California becomes the first state to create a domestic partnership law allowing gay couples to receive some protections of marriage. The Vermont Supreme Court rules that same-sex couples must be treated equally to heterosexual couples.

- 2000: A law establishing civil unions goes into effect in Vermont; it gives gay couples some of the protections of marriage. Opponents in Nebraska win a ballot initiative that prohibits the state from recognizing same-sex couples. Over the next decade, similar amendments pass in many other states.

- 2003: The Massachusetts Supreme Judicial Court rules that barring same-sex couples from marrying violates the state constitution, making it the first state to legalize same-sex marriage.

- May 17, 2004: Same-sex weddings begin in Massachusetts.

- 2004: 13 states adopt constitutional amendments banning same-sex marriage, including Arkansas, Georgia, Kentucky, Michigan and Mississippi.

- 2008: California court orders legalization. Voters subsequently overturn the ruling and limit marriage to one man and one woman. Connecticut's high court legalizes same-sex marriage after finding civil unions aren't equal to marriage.

- 2009: Gay marriage is legalized in Vermont, New Hampshire and Iowa.

- 2010: Marriage licenses become available in the District of Columbia, the first U.S. jurisdiction below the Mason-Dixon line to legalize same-sex marriage.

- 2011: New York becomes the most populous state to approve same-sex marriage.

- 2012: President Barack Obama endorses same-sex marriage; voters approve it in referendums in Maine, Maryland and Washington state.

- 2013: The Hawaii Legislature passes a bill legalizing same-sex marriage. Same-sex couples begin marrying on Dec. 2. Rhode Island, Delaware, Minnesota, New Jersey and New Mexico raise the number of states with same-sex marriage to 15.

- June 26, 2013: U.S. Supreme Court overturns a key part of the 1996 Defense of Marriage Act, allowing married same-sex couples to receive the same federal benefits as heterosexual couples. In a separate ruling, the high court clears the way for same-sex marriages to resume in California.

- 2013: Illinois Legislature legalizes same-sex marriage.

- December 2013 to May 2014: Federal or state judges in Oklahoma, Virginia, Michigan, Texas, Utah, Arkansas and Idaho find bans unconstitutional. Judges order Kentucky, Ohio and Tennessee to recognize same-sex marriages from other states. Appeals are pending or promised in all the cases.