In his blog yesterday Nate Silver wrote:
This weekend’s announcement by the former governor of Montana, Brian Schweitzer, that he would not seek that state’s Democratic nomination for Senate represents the latest in a series of favorable developments for Republicans as they seek control of the chamber.
The G.O.P.’s task will not be easy: the party holds 46 seats in the Senate, and the number will very probably be cut to 45 after a special election in New Jersey later this year. That means that they would need to win a net of six contests from Democrats in order to control 51 seats and overcome Vice President Joseph R. Biden Jr.’s tiebreaking vote. Two years ago at this time, Republicans faced what seemed to be a promising environment and could have won the Senate by gaining a net of three seats from Democrats and winning the presidency. Instead, Mitt Romney lost to President Obama, and the G.O.P. lost a net of two Senate seats.
But Montana along with West Virginia and South Dakota — two other red states where an incumbent Democrat has retired and where the Democrats have not identified a strong candidate to replace them – gives Republicans a running start. Republicans could then win three more seats from among red states like Louisiana and Arkansas, where vulnerable Democratic incumbents are on the ballot, or they could take aim at two purple states, Iowa and Michigan, where Democrats have retired. More opportunities could also come into play if the national environment becomes more favorable to Republicans (such as because of a further slide in Mr. Obama’s approval ratings). Meanwhile, while Kentucky and Georgia are possibly vulnerable, Republicans have few seats of their own to defend; unlike in 2012, they can focus almost entirely on playing offense.
A race-by-race analysis of the Senate, in fact, suggests that Republicans might now be close to even-money to win control of the chamber after next year’s elections. Our best guess, after assigning probabilities of the likelihood of a G.O.P. pickup in each state, is that Republicans will end up with somewhere between 50 and 51 Senate seats after 2014, putting them right on the threshold of a majority.
The chart below reflects our current overview of the Senate landscape, including the probability estimates. (The estimates are not based strictly on a formula but instead are best guesses, accounting for the partisan lean of each state, the quality of the prospective candidates, and approval-rating or polling data to the extent that it might be informative.) When we last conducted this exercise in February, our projection was for Republicans to win between 49 and 50 Senate seats, meaning that their standing has improved by about one seat since then.