The cascade of senators announcing or clarifying their support for same-sex marriage in the past two weeks has somewhat obscured the fact that more than half of the Democrats in the Senate backed the policy before it became fashionable. By the time Vice President Joe Biden abruptly announced his own support for the policy on May 5, 2012, nearly two-dozen senators had already signed on to a campaign begun several months earlier calling for marriage equality to be included on the Democratic Party's platform. Another eight, including independent Bernie Sanders of Vermont, expressed their support prior to 2012.
The following chart tracks each Democratic senator's position on same-sex marriage organized around when he or she made the announcement. In some cases, this date is ambiguous. After the Huffington Post listed Michigan Democrat Debbie Stabenow among those who had not formally supported the measure, for example, her office indicated that she had already done so as part of her 2012 reelection campaign. Wikipedia has a highly comprehensive page on the subject that served as a basis for many of the links in this chart, all independently verified.
The chart also includes notable Democratic leaders outside the Senate as well as Republican and independent senators who have endorsed same-sex marriage.
Update, June 18, 2013, 11:10 a.m.: Added Lisa Murkowski.
Update, April 8, 2013, 1:55 p.m.: Added Tim Johnson.
Update, April 5, 2013, 10:15 a.m.: This chart has been updated to reflect endorsements of same-sex marriage by Nelson, Heitkamp and Donnelly. To better visualize the wave of recent endorsements, 2013 is now divided into two categories, Q1 and Q2.
Update, April 3, 2013, 3:03 p.m.: Several readers have pointed out that many of the senators currently in the "2012 pre-Biden" category were co-sponsors of S. 598, the 2011 bill known as the "Respect for Marriage Act" that would have repealed the Defense of Marriage Act. Some reporters, such as the Washington Post's Dylan Matthews, count this as support for same-sex marriage. While that's a valid stance, for the purposes of this chart we are looking for explicit statements of support for same-sex marriage.
Update, April 3, 2013, 2:15 p.m.: Cardin, Schumer, Gillibrand, Boxer, Feinstein and Franken have been moved from the "pre-Biden" to "Prehistory" category to reflect earlier endorsements.