Retirements always pose a threat to congressional majorities. Before he took the helm of the Senate Democratic campaign committee in 2005, Sen. Charles Schumer (D-N.Y.) told then-Minority Leader Harry Reid (D-Nev.), "We can't let red-state Democrats retire — in '04 we lost every race where they did."
Reid agreed; Schumer subsequently won back the majority in the 2006 cycle and later helped boost the Democratic majority to 60 seats.
The Schumer-Reid exchange was detailed in Schumer's book titled, "Positively American: Winning Back the Middle-Class Majority One Family at a Time."
The Democrats who are retiring do not all hail from red states. Sen.Kent Conrad (D-N.D.) does, and Republicans are heavily favored to pick up that seat.
Republicans are also favored to win Sen. Jim Webb's (D) seat in Virginia, and their chances increase if former Virginia Gov. Tim Kaine (D) opts not to run.
Sen. Joe Lieberman (I-Conn.) is also not running for reelection. Democrats are confident they can win in this blue state, especially in a presidential election year. But in all likelihood, they will have to fight for it.
Both parties will aggressively vie for Bingaman's seat. Obama won the purple state in 2008 and George W. Bush triumphed there in 2004.
Ross K. Baker, a political scientist at Rutgers University, said, "Parties in Congress, like armies in the field, suffer casualties in two ways: They are lost in election battles and they are pulled out of the line from combat fatigue after the elections are over. The Democrats have sustained both forms of loss. The combat fatigue casualties have sized up the bloody conflicts ahead on the budget, debt extension, and entitlements and have requested to be evacuated to the rear area."
He added, "This is an ominous sign for the slender Democratic majority that can berescued only with a smashing victory at the presidential level. Obama has shown he can provide coattails and he may have to do so again."
While the retirements of the three Democrats and Lieberman have attracted many headlines and boosted the GOP's confidence, the early announcements give Democrats ample time to find strong candidates.
Sen. Patty Murray (Wash.), who heads the Democratic Senatorial Campaign Committee, has been pushing Democratic senators up for reelection in 2012 to make up their minds early in the cycle.
Republicans picked up seven seats in the 2010 cycle, including Sen. Scott Brown's (R-Mass.) win in January last year.
Democrats are targeting Brown, who faces another tough election next year. They are also targeting Sen. John Ensign (R-Nev.), whose ethics controversies pose a major obstacle to winning a third term. Republican operatives note that Brown has a $7.2 million campaign war chest and Ensign will likely face a primary challenge.
Democratic incumbents who face reelection next year include Jon Tester (Mont.), Joe Manchin (W.Va.), Sherrod Brown (Ohio), Ben Nelson (Neb.), Bill Nelson (Fla.), Debbie Stabenow (Mich.) and Claire McCaskill (Mo.).
Republicans need to win a net of at least three seats to control the Senate (four if Obama wins reelection).
This article was updated at 2:58 p.m.