South Dakota's retiring Tim Johnson became the latest Democratic senator to come out in support of gay marriage on Monday.
"After lengthy consideration, my views have evolved sufficiently to support marriage equality legislation," Johnson said in a statement. "This position doesn't require any religious denomination to alter any of its tenets; it simply forbids government from discrimination regarding who can marry whom."
The Democrat only recently announced he will not seek re-election in 2014.
Sen. Johnson's reversal follows a wave of Democratic support for marriage equality that spilled over in recent weeks as the Supreme Court took up two same-sex marriage cases. Three remaining Democratic senators--all representing conservative states--have not openly expressed their support for marriage equality: Arkansas' Sen. Mark Pryor, West Virginia Sen. Joe Manchin, and Louisiana Sen. Mary Landrieu.
Sens. Pryor and Landrieu both face re-election in 2014, while a spokeswoman for Sen. Manchin recently said that he continues to support the Defense of Marriage Act and believes "marriage is a union between one man and one woman," and will not face re-election until 2018.
Sens. Mark Kirk of Illinois and Rob Portman of Ohio are the only Republican senators who have publicly expressed their support for same-sex marriage.