Washington (CNN) -- House Appropriations Committee Chairman David Obey, a senior member of the congressional Democratic leadership, announced Wednesday that he is not seeking re-election this November.
Obey, 71, represents Wisconsin's 7th Congressional District. He was the youngest member of Congress when he was elected to his first of 21 terms in 1969.
He is the fourth-longest-serving House member, serving behind Democratic Reps. John Dingell and John Conyers, both of Michigan, and Florida Republican Rep. Bill Young.
"I think that along the way I've made a difference in the district and the state that I represent, and for the country. But there is a time to stay and a time to go, and this is my time to go," Obey said in announcing his decision.
"Frankly, I hate to do it. There is so much that needs to be done. But even more frankly, I am bone tired," he said.
House Speaker Nancy Pelosi praised Obey as a "transformational figure in the Congress."
"He is a visionary for a better life for the American people and a legislative genius: he has an ability to see around corners, anticipate challenges and opportunities and sustain a fight on behalf of what is right. For nearly a half century, he's been an indefatigable reformer," she said.
Obey said he considered retiring after the 2000 election, but he said he became so angered by the policies of the Bush administration "that I decided to stick around as long as they were here."
He also said that when he was asked in the past if he was contemplating retirement, he would tell reporters he didn't want to retire until Congress passed health care reform.
"Well, now it has, and I can leave with the knowledge that thanks to Speaker Pelosi and President Obama and so many others, we got the job done. I haven't done all of the big things that I wanted to do when I started out, but I've done all of the big things that I'm likely to do," he said.
Obey won the 2008 election with 61 percent of the vote.
This term, he was expected to face Republican challenger Sean Duffy, the district attorney of Ashland County. Earlier this year, Time magazine named Duffy as one of 10 GOP contenders capable of providing a Scott Brown-like upset, referring to the Republican's surprise win in the Massachusetts Senate election.
Duffy also appeared on the sixth season of MTV's "The Real World."
The National Republican Congressional Committee issued a statement saying Obey was facing "the race of his life."
"That is why it is understandable that the architect of President Obama's failed stimulus plan has decided to call it quits. It is ironic that a congressman who became infamously known for his short temper and angry tirades on the House floor, is going out with such a whimper," NRCC communications director Ken Spain said in a statement.
The nonpartisan Rothenberg Political Report had characterized the district as "leaning Democratic."
The Democratic Congressional Campaign Committee praised Obey as a "tireless voice for progress on behalf of his constituents in Wisconsin and middle class families across America" and disputed the notion that his job was at risk.
"His success in writing a new code of ethics for the House of Representatives and ensuring passage of our historic health insurance reforms will stand as Chairman Obey's proud legacy," DCCC Chairman Chris Van Hollen said in a statement.
"Chairman Obey would have won re-election again had he run. We are confident that a Democrat who shares Chairman Obey's commitment to making progress for Wisconsin's middle class families will succeed him as the next Representative of Wisconsin's 7th Congressional District."
Obey also expressed confidence that he would have held on to his seat.
"Let me put it this way -- I have won 25 elections," Obey said. "Does anybody really think I don't know how to win another one? Or, for that matter, has anyone ever seen me walk away from a fight in my life?"
"The fact is there isn't a chance of a snowball in Hades of that progressive congressional district electing someone who is a poor imitation of George Bush's policies on a bad day," Obey added to applause.
Obey said he believes the job of a good politician is to be "used up" fighting for causes one believes in, and then stepping aside to let someone else carry on the battle.
"Well today folks, I feel used up," he said.
Obey said he wasn't sure what he planned to do next, but joked, "All I do know is that there has to be more to life than explaining the ridiculous accountability-destroying rules of the United States Senate to confused and angry and frustrated constituents."
CNN's Deirdre Walsh contributed to this report