Similar to Democrats last cycle, retirements dramatically shifted races from safe to vulnerable in 2008, as the exits of GOP Sens. Pete Domenici (N.M.) and John Warner (Va.) led to easy Democratic victories in open seats.
In January 2005, President George W. Bush was inaugurated for his second term after a tough re-election race and Democratic Sens. Bill Nelson (Fla.) and Ben Nelson (Neb.) were thought to be at risk. But the environment dramatically shifted against the president by the time 2006 midterm elections arrived, and both Nelsons won re-election by 20 points. This cycle, Roll Call Politics rates both of their re-election races as Tossups.
In 2006, Republicans lost six Senate seats, and the majority, but the biggest surprise might have been in Ohio, where Sen. Mike DeWine (R) was thought to be safe but lost to Rep. Sherrod Brown (D) by 22 points. On the Democratic side, Sen. Paul Sarbanes (D) was at no risk of losing in Maryland, but his retirement made Democrats work to hold his seat as then-Rep. Benjamin Cardin (D) eventually defeated Michael Steele (R).
Early in the 2004 cycle, Sen. John Breaux (D) was considered safe, but he retired and Vitter won the seat, 51 percent to 29 percent. Sen. Ben Nighthorse Campbell (R) was safe in Colorado, but he retired and Ken Salazar (D) won the competitive open seat.
Early in the 2002 cycle, Sen. Phil Gramm (R-Texas) was viewed as safe, but after he retired, John Cornyn had to work to keep the competitive open seat in GOP hands.
Democrats start this cycle dramatically on defense (the party has 23 seats up compared with 10 for the Republicans), and with public polling showing multiple vulnerable incumbents, only time will tell which party gains seats in November 2012.
“Ratings are similar to polls; they are snapshots in time but not necessarily harbingers of things to come,” National Republican Senatorial Committee Executive Director Rob Jesmer said.
Former Sen. Scott Brown, R-Mass., candidate for U.S. Senate in New Hampshire, holds his hand over his heart during the singing of the national anthem as he waits to take the stage for his town hall campaign rally with Sen. John McCain at the Pinkerton Academy in Derry, N.H., on Monday, Aug. 18, 2014.