Measuring probabilities on the Senate map is partly a product of determining who's in and who's out. Democrats will be defending 21 seats in 2014; 14 Republican seats are up. Four long-time Senators have now announced they won't be seeking re-election: Massachusetts' John Kerry, West Virginia's Jay Rockefeller, Georgia's Saxby Chambliss, and Iowa's Tom Harkin.
Which other Senate veterans might join them and opt out in 2014? South Dakota Senator Tim Johnson is the most vulnerable member on the "most likely to retire" list. Johnson had surgery in 2006 for bleeding in his brain, which slowed his speech, but made a comeback with a 2008 win.
In December, after former Republican governor Mike Rounds announced he planned to run for the seat, Johnson told reporters, "I fully intend to put together a winning campaign in the weeks and months ahead." But later he backtracked, saying, "If I run again, I will run a strong campaign is what I meant. But only if I run again, and it's far too soon to make that statement." He has said he will decide early this year.
Then there's the Democrat who will be 90 in 2014, but insists he has no intention of retiring. New Jersey Senator Frank Lautenberg already has a high-profile challenger in Newark mayor Cory Booker, who leads Lautenberg in recent polls by double digits.
Last week, Lautenberg chided Booker in an interview with the Philadelphia Inquirer, saying, "I have four children, I love each one of them. I can't tell you that one of them wasn't occasionally disrespectful, so I gave them a spanking and everything was OK."
On Sunday, the junior senator from New Jersey, Robert Menendez, tried to steer clear of the Lautenberg-Booker spat, laughing, "That election is next year, and all of the back-and-forth now is something I'm really not focused on."
Also on the Democratic side, Michigan Senator Carl Levin, the longest-serving senator in the state's history, will be 80 in 2014 and could retire. Illinois' Dick Durbin is just 68, but is also a possible retirement. Senate Majority Leader Harry Reid has encouraged members to announce their plans early, to avoid an Evan Bayh-like situation. In 2010, the former senator from Indiana announced he would retire just days before the filing deadline, leaving the party scrambling to recruit a competitive candidate.
On the Republican side, the most likely retirements are Wyoming Senator Mike Enzi and Mississippi Senator Thad Cochran. Enzi told Politico in November, "I'm running hard." But he toyed with a retirement in 2008, and didn't decide until March of that year he would run, and could make a game-time decision this time as well. Cochran, who has served in the Senate since 1978, has said it's "too early to decide."
For more on what the Senate retirements mean for competitive primaries, check out today's First Read.