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.
Source: Business Insider. Read full article. (link)