Sen. Tim Johnson (D-SD)

I’m simply not good at predicting these US Senate turnovers on this issue; not quite as good as Chris Cillizza, anyway. He had Tim Johnson listed first, back when there were six. I had thought not running for re-election immunized Johnson from having to take a stand on this issue; Cillizza claimed not having to face South Dakota voters might free him to speak out.

Tim Johnson, who is retiring and therefore won’t face South Dakota voters again, announced Monday that he now backs marriage equality.

Sen. Tim Johnson (D-S.D.) joined the stampede of marriage-equality supporters today, leaving just three Senate Democrats who have not voiced support for same-sex marriage.

“After lengthy consideration, my views have evolved sufficiently to support marriage equality legislation,” Johnson said in a statement to Talking Points Memo. “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.”

Johnson is the 54th member of the Senate to back marriage equality. On Friday, Democratic Sens. Joe Donnelly (Ind.) and Heidi Heitkamp (N.D.) became the latest Senate Democrats to endorse marriage equality as the Supreme Court considers two landmark same-sex marriage cases.

Unlike other recent conversions, Senator Johnson explicitly backs “marriage equality legislation” and is not simply supportive of the cases before the Supreme Court. It’s an interesting distinction as Gay Inc. considers whether to press for the Respect For Marriage Act, which repeals DOMA legislatively, in their current strategy.

Unfazed and unmoving in their opposition are Sens. Mary Landrieu (La.), Joe Manchin (W.Va.) and Mark Pryor (Ark.). Last week Mark Pryor said “put me down in the undecided category. As for Mary Landrieu:

In a recent interview with CNN, Landrieu suggested that while she personally believes same-sex couples should be able to marry — saying “people should love who they love and marry who they want to marry” — she’s unable to officially take that position because her state has passed a constitutional amendment banning same-sex marriage. Landrieu is up for re-election next year in a state that’s consistently “red” in presidential elections.

“My state has a very strong constitutional amendment not only against gay marriage but against gay partnerships,” Landrieu said. “So I’m looking at the people of Louisiana trying to represent their interests,” she said.

Leading or following, that’s always the dilemma, isn’t it, Mary?