Levin, 78, was first elected to the Senate in 1978 and is the longest-serving senator in Michigan's history. He said in a statement the decision was "extremely difficult because I love representing the people of Michigan" and "fighting for the things that I believe are important for them."
Levin is the sixth member of the Senate to announce his retirement, creating an open seat for Democrats in a state that has backed President Barack Obama twice but where Republicans hold the governor's office. Democrats, who control 55 seats in the Senate, have to defend open seats in West Virginia, Iowa and New Jersey in the aftermath of three other retirements and will try to hold onto 21 seats in next year's elections.
Rep. Gary Peters, a Democrat who represents suburban Detroit, has been viewed as a potential Senate candidate, while Rep. Justin Amash, a Republican who represents a district that includes Grand Rapids, is also considering a run.
Levin's announcement comes just days after he shepherded Defense Secretary Chuck Hagel's nomination through a bruising Senate confirmation fight. The Armed Services chairman often found himself at odds with some of the newer Republicans on the committee, including freshman Sen. Ted Cruz of Texas. Levin pointedly pushed back when Cruz insinuated that Hagel had taken money from extreme or radical groups, and he defended the president's nominee.
Levin, who has served on the committee for nearly three decades, became chairman in 2007 after Democrats regained the majority in the Senate. He has worked closely with Sen. John McCain of Arizona, then the top GOP committee member, in ensuring that the Congress produces an annual defense policy bill, which it has done for more than half a century. The panel is one of the few committees to successfully produce a bipartisan authorization bill.
Levin clashed with the Obama administration in 2011 over provisions in the bill on the handling of terror suspects and detention policy, but ultimately prevailed in establishing various policies.
Closer to home, Levin has been an outspoken advocate for his home state auto industry and manufacturers. When General Motors and Chrysler faced potential collapse in 2008, Levin pressed his fellow members and the incoming Obama administration to support the companies with billions of dollars in loans. The automakers have since rebounded.