By NATE SILVER

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.
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