A surprise decision by popular former Democratic Gov. Brian Schweitzer not to run for Montana's open Senate seat in 2014 has made control of the Senate a complete toss up.

The New York Times' Nate Silver thinks that Republicans are right on the cusp of gaining enough seats in the 2014 election to hold a Senate majority in 2015. Silver projects a "best guess" of the GOP holding either 50 or 51 seats. The difference is crucial, because theoretically Vice President Joe Biden would be the tiebreaking vote in a majority-vote situation.

Republicans will likely have to win six seats to get to 51 and take control of the Senate. Such a swing could be huge, because it would mean that Republicans would control both the House and Senate while President Barack Obama finishes the last two years of his second term in the White House.

The three likely pickups come from vacancies in Montana, West Virginia, and South Dakota, where Democrats are all retiring. Meanwhile, Silver projects that seats in North Carolina, Lousiana, and Arkansas — all with incumbent Democrats — are toss-ups. A win in Alaska, which Silver considers the "tipping point" state, would give Republicans 51 seats. Right now, Silver considers the seat held by Sen. Mark Begich (D) to lean Democratic.

Meanwhile, Kyle Kondick at the University of Virginia also says that a GOP-controlled Senate is now "a lot easier to imagine."

Here's how the analyst, who is a member of Larry Sabato's "Crystal Ball" team, has the Senate situation shaping up: