The basic odds make it fairly unlikely that the Democrats will maintain their Senate majority. They only hold a narrow 53-47 edge after the 2010 midterms, and the party must defend 23 seats in 2012, compared to just ten for Republicans. Their troubles only increased when moderate Democrats hailing from conservative states—Ben Nelson and Kent Conrad as the most notable—decided that now was the time to retire, all but ceding their spots to the GOP. Every scenario looked doom and gloom for their chances. But then Republicans decided to sabotage those odds. First Olympia Snowe announced her retirement, after growing tired of her party's partisan rancor. Her seat is expected to go to the independent—but Democratic friendly—candidate Angus King. Last week, Indiana Republicans booted out longtime Senator Dick Lugar in favor of a Tea Party challenger, while Nebraskans selected the right wing candidate in their primary earlier this week.
Polling maestro Nate Silver of Five Thirty Eight is out with a new projection of the Senate breakdown post-November. If Silver's estimation is right, it will be a nail biter of an election night:
Currently, we project the most likely outcome to be Republicans winning 50 seats, Democrats 49, and Mr. King the seat in Maine. Under those circumstances, the Democrats would retain control of the Senate if Mr. King caucused with them and President Obama won re-election, making Vice President Joseph R. Biden Jr. the tiebreaking vote. Otherwise, Republicans would control the chamber.
As Silver notes, contested states often tend to swing in one party's favor en masse when the election arrives, but it is not entirely implausible that the presidential election (and thus the vice president's tie-breaking vote) could decide which party holds the Senate majority in 2013.