Clinton's began towards supporting marriage equality in 1999, when she told a group of donors that she supported domestic partnerships. That's a legal arrangement that provides gay and lesbian couples with a limited number of rights, without coming close to equalling marriage.

"I think a marriage is as a marriage has always been, between a man and a woman," she said in 2000. "I don't support gay marriages, but I do support extending benefits to couples, domestic partner benefits." At the time, no states allowed gay and lesbian couples to marry, and public opinion was strongly against marriage equality.

Several years later in 2006, Clinton defended the federal Defense of Marriage Act, which prevented federal recognition of marriage equality, as a strategic move. Without DOMA, she told a group of gay politicians, the country might have faced a constitutional marriage ban, which would be much more difficult to overturn than a simple federal law.

At a forum in 2007, Clinton avoided discussion of marriage and focused instead on civil unions, a legal formulation that purports to provide similar benefits to marriage but still does not carry the same rights and obligations. Clinton told the audience,

I prefer to think of it as being very positive about civil union. ... And I am absolutely in favor of civil unions with full equality, full equality of benefits, rights and privileges. And I’ve also been a very strong supporter of letting the states maintain their jurisdiction over marriage.

In 2011, New York legalized the freedom to marry. “I've always believed that we would make progress because we were on the right side of equality and justice," Clinton said, the closest she'd come to date to advocating for marriage equality.

During this time, she was serving as Secretary of State, an role which prevented her from participating in domestic political issues.

Several months later, she released a video in which she told a global audience, “Gay rights are human rights, and human rights are gay rights."

It was on March 18 of 2013 that Clinton firmly stated her support for marriage equality, a little over a month after she stepped down from her position as Secretary of State. 

But there were some tense words exchanged in 2014, when Clinton appeared on the NPR program Fresh Air. Host Terry Gross pressed Clinton about when her position shifted from opposing marriage equality to supporting it, and "I don’t think you are trying to clarify," Clinton said. "I think you are trying to say that I used to be opposed [to gay marriage], and now I am in favor, and I did it for political reasons. And that’s just flat wrong. ... I have a strong record. I have a great commitment to this issue and I am proud of what I’ve done and the progress we're making."

Clinton's evasive response to Gross raised some eyebrows about her commitment to the issue. Many Clinton supporters believed that she had privately supported marriage equality for years, and her reluctance to answer Gross' questions indicated that she was resistant to offer specifics about how she came to change her mind on the issue.

In 2014, Clinton told CNN that "I think 'evolved' is the word that a lot of people have used." She continued:

It fits me as well as it fits others. In large measure based on the experiences that I had with so many people who I knew and cared about. ... And it really became very clear to me that if we're going to support marriage in our country, it should be available to everyone regardless of who they love and that this marriage equality issue is a great human rights issue ... So yes, I evolved over time and I'm very, very proud to state that I'm a full supporter of marriage equality right now.

The U.S. Supreme Court is expected to rule before the summer of 2015 on marriage equality. The Justices could issue a ruling that legalizes marriage equality nationwide. Or they could uphold the marriage bans that still exist in numerous states.