The decision by the Indiana Democrat, who was in a strong position to win a third term in November in his Republican-leaning state, makes it likelier that the 59 votes that give Democrats command of the 100-seat Senate will dwindle, particularly given the intense voter anger directed at incumbents.
Democrats are still recovering from Republican Scott Brown's stunning January upset to take the late Edward Kennedy's long-held Senate seat in Massachusetts. Brown's win cost the Democrats their 60-seat supermajority, imperiling the party's drive for passing sweeping health care reform, Mr. Obama's top domestic priority. Senate rules require 60 votes to overcome procedural maneuvers to endlessly extend debate and block a final vote on legislation.
continues a recent exodus from Congress among Democrats, including veteran Democrats Christopher Dodd of Connecticut and Patrick Kennedy of Rhode Island.
North Dakota's Democratic Sen. Byron Dorgan also is retiring, and his party doesn't have anyone to challenge the Republican, Gov. John Hoeven. Democrats also failed to recruit their top candidate in Delaware. Vice President Joe Biden's son, Beau Biden, eschewed a run against Republican Mike Castle for the seat long held by his father.
Even some incumbents are threatened, including Senate Majority Leader Harry Reid of Nevada who trails all his potential Republican opponents in recent polls.
Several Republican senators have also decided not to seek re-election - in Missouri, Florida, New Hampshire and Ohio - but early polls show the Republicans favored to keep these seats in their column in the November election when control of Congress will be at stake.
The Republicans would have to win every race in November to get close to taking back control of the Senate. But Democrats, who were expected to pick up seats only a few months ago, are now expected to lose several - a sign of how tough the electoral environment is for the party of power, reports CBS News correspondent Nancy Cordes.
Democrats have a 255-178 edge in the House of Representatives, with two vacancies from Democratic-held seats. But there are 49 Democrats from districts Republican presidential candidate John McCain won in 2008, placing them among the most endangered House Democrats.
With the public upset over job losses, spiraling federal deficits and spending, huge bonuses awarded to executives of bailed-out financial institutions, and Washington's yearlong preoccupation with health care, one need look no further than recent polls to gauge the poisonous political atmosphere facing members of Congress seeking re-election:
-In an Associated Press-GfK poll in mid-January, just 32 percent approved of how Congress was handling its job, including just 4 percent strongly approving, though Democrats got higher marks than Republicans. People were split about evenly over whether they wanted their own members of Congress to be re-elected, an unusually poor showing. And while nearly everyone named the economy as the most important issue, just one in five considered the economy in good shape.
-A CBS News/New York Times poll in early February found 81 percent saying it's time to elect new people to Congress, with just 8 percent saying most members deserve re-election.
The departure of Bayh, 54, sent deeper shock waves than most. Telegenic and on the list of potential running mates for the past two Democratic presidential tickets, Bayh is known more for the moderate tone of his politics than for any particular legislative achievements, and his parting words had a notably plaintive tenor.
"I love working for the people of Indiana, I love helping our citizens make the most of their lives, but I do not love Congress," Bayh said Monday at a news conference in Indianapolis, where he was joined by his wife and two sons.
He also lambasted the bitter divide between Democrats and Republicans in Washington, saying, "I value my independence. I am not motivated by strident partisanship or ideology." He added that he wanted to work in the private sector, perhaps running a business, university or charity, for "solutions not slogans, progress not politics."
Bayh, 54, whose father, Birch, served three terms in the Senate, said he believed he would have been re-elected this November, despite "the current challenging environment." He had raised nearly $13 million for his campaign and faced little-known Republican opposition until national Republicans recruited former Sen. Dan Coats to enter the race.
Republicans saw a more partisan motivation in Bayh's departure.
"The fact of the matter is Senator Evan Bayh and moderate Democrats across the country are running for the hills because they sold out their constituents and don't want to face them at the ballot box," Michael Steele, chairman of the national Republican Party, said in a written statement regarding the massive $787 billion stimulus bill enacted a year ago and other measures.
The Democrats will have to scramble to find a replacement candidate for Bayh's seat. Friday is the filing deadline for the May primary, although the party would have until June 30 to select a replacement candidate.
The toll taken by grinding political conflict and gridlock that Bayh described rang true to Dave Nagle, a Democratic political activist and former congressman from Iowa.
"It's not like going to work every day, it's like going to war," Nagle said in an interview. "You can only hear the bugle on the Hill so many times, then you grow tired of it. It just isn't worth it."
The wrath of angry voters has had a decidedly bipartisan impact, with recent retirement announcements coming from Republican Rep. Lincoln Diaz-Balart and other Republican House members from Michigan, Indiana, Arkansas and Arizona.
"Whether you're a Republican or a Democrat, you've probably had some very nasty town hall meetings lately, and most normal human beings don't enjoy being yelled at," said Jack Pitney, a political scientist at Claremont-McKenna College in Claremont, California. "Democrats stand to lose more than Republicans because they're the in party, but Republicans are catching some of this too."