Democratic Sen. Carl Levin, 78, announced Thursday that he will not run for a seventh term in 2014.
Levin, chairman of the Senate Armed Services Committee, called it an "extremely difficult" decision and stated that he'll better serve his Michigan constituents in the next two years of his term by not being distracted by a campaign.
"This decision was extremely difficult because I love representing the people of Michigan in the U.S. Senate and fighting for the things that I believe are important to them," he said in a statement.
During the remainder of his time in Washington, Levin said he plans to focus on his continued push for stricter rules against tax avoidance, saying tax loopholes that allow off-shore accounts are "a major drain on our treasury." He also wants to fight to boost manufacturing and address what he sees as serious flaws in campaign finance laws.
Further, Levin expressed concern about the government's fiscal problems on the military and showed a strong desire to ensure a "rapid transfer of responsibility for Afghan security to the Afghans."
"These issues will have an enormous impact on the people of Michigan and the nation for years to come, and we need to confront them," he continued. "I can think of no better way to spend the next two years than to devote all of my energy and attention to taking on these challenges."
In a statement, President Barack Obama said Levin is a "true champion for all those who serve and his tireless work will be missed not just in his home state of Michigan, but by military families across our country."
"If you've ever worn the uniform, worked a shift on an assembly line, or sacrificed to make ends meet, then you've had a voice and a vote in Senator Carl Levin," he said. "No one has worked harder to bring manufacturing jobs back to our shores, close unfair tax loopholes, and ensure that everyone plays by the same set of rules."
First elected in 1978, Levin is the longest-serving U.S. senator in Michigan's history. Before joining the Senate, Levin worked as a public defender in Detroit, where he grew up, and was elected to the Detroit City Council in 1969.
His older brother, Sander Levin, serves in the U.S. House of Representatives as the top Democrat on the House tax writing committee. The older Levin still plans to run for re-election next year, according to his spokesman.
Sen. Michael Bennet, chairman of the Democratic Senatorial Campaign Committee, said Levin has been a "model public servant" for more than three decades.
"I'm particularly thankful for his leadership on behalf of our men and women in uniform," he said in a statement. "I am confident that we will recruit a great Democratic leader who will continue to fight for the values and priorities Senator Levin advocated for all these years. We fully expect to keep Michigan blue in November 2014."
Levin has won most of his elections by a wide margin, including his re-election bid in 2008 when he won with 63% of the vote.
He's now the fourth Democratic senator to announce that he'll step down after the next term. Fellow Democratic senators Jay Rockefeller of West Virginia, Tom Harkin of Iowa, Frank Lautenberg of New Jersey all announced they would retire from the Senate instead of making bids for re-election next year.
Other Democratic senators who may possibly retire include Tim Johnson of South Dakota and Dick Durbin of Illinois.
On the Republican side, Saxby Chambliss of Georgia and Mike Johanns of Nebraska also decided against a run.
At this point, Democrats will be defending 21 of the 35 Senate seats up for election in 2014. They currently have an advantage with 55 seats in the Senate (including two independents who caucus with them) to Republicans' 45 seats.
CNN's Ashley Killough, Paul Steinhauser, Ted Barrett, Adam Levine, and Deirdre Walsh contributed to this report.