The 2012 senate arithmetic alone represents a challenge to Democrats to hold on to the upper chamber. But in the last 5 weeks, several Democrats have announced that they plan to retire, making what was a fighting chance for a GOP senate takeover into a better than 50-50 proposition.The Hill: An early spate of Democratic Senate retirements has put Republicans in solid shape to retake the majority in the upper chamber next year.The first edition of The Hill's 2012 race ratings puts five Democratic-held seats in the toss-up column. Republicans need a net gain of at least three seats to win the Senate.Sens. Jon Tester (D-Mont.) and Ben Nelson (D-Neb.) are the two incumbents that top of the list of vulnerable Senate Democrats in 2012. And, thanks to retirements, another three Democratic-held seats are toss-ups -- the ones held by Sens. Kent Conrad (N.D.), Jeff Bingaman (N.M.) and Jim Webb (Va.).Expected primary challenges to Dick Lugar (IN), Orrin Hatch (UT), and Charles Grassley (IA) may...(Read Full Post)