Photo by Chip Somodevilla/Getty Images 
 
 Sen. Tom Carper is the most popular, safe politician in my birth state of Delaware. In my lifetime, the closest he came to a fight was the 2000 race for U.S. Senate, when national Democrats tasked him with defeating Republican Sen. Bill Roth. (You have a Roth IRA? Thank Bill Roth.) Pundits, in those pre-Nate Silver dark ages, thought it would be a toss-up. Carper beat Roth by 12 points, and went on to win two more terms with more than two-thirds of the vote. In 2012 he secured his job for six years, running seven points ahead of the Obama-Biden ticket. 
 
 
 
 David Weigel David Weigel is a reporter for Bloomberg Politics. 
 
 This all made Carper a strange hold-out on gay marriage. For weeks, when you asked his office what he thought of marriage equality you got this statement. 
 
 
 
 Sen. Carper was proud to support Delaware’s efforts to enact Civil Union legislation and earlier this month he joined 211 of his Congressional colleagues in co-signing the Amicus brief that urges the Supreme Court to invalidate Section 3 of DOMA. He has also said that he would vote to repeal DOMA. He also opposed President Bush’s attempt to enact a constitutional amendment to define marriage as between one man and one woman. Like many Americans including Presidents Obama and Clinton, Sen. Carper’s views on this issue have evolved, and continue to evolve. He continues to give this issue a great deal of consideration. 
 
 
 
 
 
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 As we've seen, the word "evolve" is a Hardy Boys-worthy hint: No one says he's "evolving" and doesn't eventually change his mind. But when would Carper do it? He's 76, and part of a generation that's been slow to embrace gay marriage, but there was no political risk for him embracing the Delawarean vice president's position. 
 
 
 
 He finally got there today, on Facebook: 
 
 
 
 As our society has changed and evolved, so too has the public's opinion on gay marriage – and so has mine. I pray every day for God to grant me the wisdom to do what is right. Through my prayers and conversations with my family and countless friends and Delawareans, I've been reminded of the power of one of my core values: the Golden Rule. It calls on us to treat others as we want to be treated. That means, to me, that all Americans ultimately should be free to marry the people they love and intend to share their lives with, regardless of their sexual orientation, and that's why today, after a great deal of soul searching, I'm endorsing marriage equality. 
 
 Carper's evolution reduces the Senate's number of gay marriage Democratic hold-outs to seven: Indiana Sen. Joe Donnelly, North Dakota Sen. Heidi Heitkamp, South Dakota Sen. Tim Johnson, Louisiana Sen. Mary Landrieu, West Virginia Sen. Joe Manchin, Florida Sen. Bill Nelson, and Arkansas Sen. Mark Pryor. In what order will they evolve? To know this we need to put them into teams. 
 
 Senators retiring next year: Johnson 
 
 Senators up for re-election next year: Landrieu, Pryor 
 
 Senators not up for re-election until 2018: Donnelly, Heitkamp, Manchin, Nelson 
 
 Senators from states that have voted for Obama at least once: Donnelly, Nelson ||||| Sens. Heidi Heitkamp (D-N.D.) and Joe Donnelly (D-Ind.), both hailing from deeply red states, announced on Friday their support for gay marriage. 
 
 "In speaking with North Dakotans from every corner of our great state, and much personal reflection, I have concluded the federal government should no longer discriminate against people who want to make lifelong, loving commitments to each other or interfere in personal, private, and intimate relationships," Heitkamp said in a statement. "I view the ability of anyone to marry as a logical extension of this belief. The makeup of families is changing, but the importance of family is enduring." 
 
 "In recent years, our country has been involved in an important discussion on the issue of marriage equality," Donnelly said on Facebook. "While serving in the House of Representatives, I had the opportunity to act on a core belief of mine: we are a stronger country when we draw on the strengths of all Americans. I voted to repeal ‘don't ask, don't tell’ and was an original supporter of the bill that would make it illegal to discriminate against someone in the workplace because of their sexual orientation." 
 
 "It is also for that reason that I oppose amending either Indiana’s or our nation’s constitution to enshrine in those documents an 'us' and a 'them,' instead of a 'we,'" he continued. "With the recent Supreme Court arguments and accompanying public discussion of same-sex marriage, I have been thinking about my past positions and votes. In doing so, I have concluded that the right thing to do is to support marriage equality for all." 
 
 Heitkamp and Donnelly join several other Democratic senators who have recently made statements backing marriage equality, including Sen. Tom Carper (D-Del.) and Bill Nelson (D-Fla.). Support from Heitkamp and Donnelly is particularly notable, however, because both come from states that voted for Mitt Romney in 2012 and generally elect Republicans. However, neither face reelection until 2018, and if public opinion continues its dramatic shift in favor of same-sex marriage, the issue may become less controversial. 
 
 Only four remaining Democratic senators have not publicly backed gay marriage: Mark Pryor (D-Ark.), Joe Manchin (D-W.Va.), Mary Landrieu (D-La.) and Tim Johnson (D-S.D.). Landrieu and Pryor face re-election in 2014 in Southern red states. Johnson is retiring in 2014, but there is a movement to draft his son to run for his seat. Manchin, like Donnelly and Heitkamp, is not up for reelection until 2018.
– Add two more senators to the list of gay-marriage "evolvers"—North Dakota's Heidi Heitkamp and Indiana's Joe Donnelly said today that they support same-sex marriage, reports the Huffington Post. Both are Democrats from conservative states and both are safe from re-election campaigns until 2018. "The makeup of families is changing, but the importance of family is enduring," said Heitkamp in a statement. Together with yesterday's flip by Florida's Sen. Bill Nelson, the moves leave only four Democratic senators who have not publicly voiced support for gay marriage. If you're a bit amazed by how quickly senators have been "evolving" on this issue, you're not the only one. But Nate Silver decided to take things a step further, as usual, and try to come up with a mathematical model to explain why, for his latest blog post in the New York Times. The key, says Silver, is public support for gay marriage in many states growing from a slight majority to a clear one. But he expects the pace to slow down greatly at this point. "The remaining senators who have not taken the opportunity yet may have good political reasons for it, and may wait some time before they do," he writes.