(CNN) -- Connecticut Sen. Chris Dodd, one of the most powerful Democrats on Capitol Hill, announced Wednesday that he will not seek a sixth term in November.
"I have been a Connecticut senator for 30 years," the 65-year-old, white-haired senator told reporters and supporters in East Haddam, Connecticut, where he lives with his wife and two young daughters. His wife, Jackie, held one of the couple's daughters and stood behind him as he spoke.
"I'm very proud of the job I've done and the results delivered. But none of us is irreplaceable. None of us is indispensable," he said.
Dodd was first elected to the Senate in 1980, and had won congressional elections in his state since 1974. Recently, however, he has been considered one of the most vulnerable Democrats seeking re-election in 2010, and has trailed likely Republican challengers in recent polls, even though Connecticut typically leans Democratic.
He acknowledged his more precarious political standing in his remarks Wednesday, as well as his tumultuous last year.
"Over the past 12 months, I've managed four major pieces of legislation through the United States Congress, served as chair and acting chair of two major Senate committees, placing me at the center of the two most important issues of our time -- health care and reform of financial services," he said.
"I lost a beloved sister in July, and in August, Ted Kennedy. I battled cancer over the summer, and in the midst of all of this, found myself in the toughest political shape of my career," Dodd said. "None of these events or circumstances, either individually or collectively, is the cause for my decision not to seek re-election."
He added: "Yet, together these challenges have given me pause, to take stock and to ask questions that too few of us in elected public life ever do -- why am I running?"
Dodd has been criticized for ties to the financial industry, which is particularly influential in Connecticut. The senator was criticized last year for his role in handing out big bonuses at American International Group, after the insurance giant received taxpayer bailout money. AIG's Financial Products unit is based in Connecticut.
Dodd initially denied having anything to do with paying out millions in bonuses at AIG, then later acknowledged his role.
Dodd, who is chairman of the Senate Banking Committee, acknowledged that he was responsible for language added to the stimulus bill to ensure that existing contracts for bonuses at bailout recipients, such as AIG, were honored.
Soon after that, Dodd acknowledged that his poll numbers had slipped. Rumors about Dodd's retirement have swirled around Washington for months.
Connecticut Attorney General Richard Blumenthal, among the state's most popular Democrats, planned to announce his candidacy for Dodd's seat later Wednesday, a highly placed Connecticut Democrat told CNN.
A senior Democratic source involved in Dodd's campaign said Democratic party officials had become convinced that Dodd's re-election bid was "virtually unwinnable for us."
The source and others have said there have been quiet conversations among party officials for some time about Dodd stepping aside, but they said it does not appear that party leaders specifically asked him to do so.
Democratic sources said Dodd began making calls to associates about his decision Tuesday. However, a source with knowledge of the conversations told CNN that he did not call Senate Majority Leader Harry Reid until after midnight, after the news had already been reported.
"Let me be clear," Dodd said Wednesday. "I'm very aware of my present political standing here at home in Connecticut, but it's equally clear that any certain prediction about an election victory or defeat nearly a year from now would be absurd."
Dodd's news conference comes a day after another Democratic senator, Byron Dorgan of North Dakota, announced that he will not seek re-election in November.
Dorgan's seat, unlike Dodd's, was considered safe for Democrats, who are trying to hold on to their 60-seat filibuster-proof Senate majority. Most observers believe North Dakota will now become a top GOP target. Some top Republicans are urging that state's popular GOP governor, John Hoeven, to run for the seat.
Republican National Committee chairman Michael Steele said Tuesday that Dorgan's decision "highlights just how vulnerable both Senate and House Democrats have become since deciding to walk in lockstep with President Obama's government-run policies."
Rob Simmons, a Republican who had been leading Dodd in recent polls, said Thursday that he is still in the race.
Simmons said Wednesday that any Democratic nominee will have to defend "failed Democratic policies."
Dodd's retirement will, at least for the moment, silence his family's long-powerful voice in Connecticut politics. His father, Thomas Dodd, represented the state in the U.S. House from 1953 to 1957, and in the U.S. Senate between 1959 and 1971.
Shortly after the announcement, President Obama said in a statement that Dodd has "worked tirelessly to improve the lives of our children and families, support good jobs for hard-working Americans, and keep our nation strong and prosperous, building a remarkable record of achievement for the people of Connecticut and our country."
Vice President Joe Biden said Dodd will "be long recognized as one of the most significant senators of my generation."
He added, "I believe the nation will miss his wisdom, wit and compassion. I count myself lucky because I know he's not going too far and will always be a source of advice and counsel."
Sen. Joe Lieberman, Dodd's fellow senator from Connecticut, said Dodd has been an "unusually skillful and productive legislator."
He "leaves a great legacy of accomplishment that has improved the lives of millions of American families and children," Lieberman, an independent, said.
Reid said in a statement that he knew how "much of an honor" it was for Dodd to serve the people of Connecticut, and "how truly difficult this decision was for him to step away."
CNN's John King, Dana Bash and Ed Hornick contributed to this report.