* Becomes sixth senator to announce retirement plans
* Democrats expected to retain his seat
* Was among most liberal senators
By Thomas Ferraro
WASHINGTON, March 7 (Reuters) - Democratic U.S. Senator Carl Levin of Michigan, a leading voice on national security who opposed the Iraq War and has fought corporate abuse during more than three decades in the chamber, said on Thursday he would not seek re-election next year.
"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," Levin, 78, said in a statement issued by his office.
In addition to chairing the Armed Services Committee since 2007, Levin heads the Permanent Subcommittee on Investigations.
With Levin at the helm, the panel has investigated issues in recent years ranging from money-laundering and tax shelters to causes of the 2007-2009 U.S. financial crisis.
"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," President Barack Obama said in a statement. "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."
Levin became the sixth senator to announce he would not seek re-election next year, following Democrats Tom Harkin of Iowa, Frank Lautenberg of New Jersey and Jay Rockefeller of West Virginia, and Republicans Saxby Chambliss of Georgia and Mike Johanns of Nebraska.
Overall, 35 of the 100 Senate seats are up for election in 2014, 21 now held by Democrats, 14 by Republicans.
The Republicans need a net gain of six seats to take control of the chamber, now held by Democrats, 55-45.
SEAT MOVES TO 'LEAN DEMOCRAT'
Levin's announcement he would retire from the Senate when his current term expires in January 2015 had been somewhat anticipated.
Shortly after the announcement, the nonpartisan Cook Political Report, which tracks congressional races, moved his seat in the 2014 election to "Lean Democrat" from "Solid Democrat."
Leading contenders for the seat include second-term Republican Representative Justin Amash, who expressed an interest in the past in running for the Senate, and Democratic Representative Gary Peters, first elected to the House of Representatives in 2008.
Levin said in his statement he would rather spend the next two years tackling tough issues, like fiscal pressures on military readiness and the need to plug corporate tax loopholes, than running for a seventh Senate term.
"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," Levin said.
Known for his rumpled appearance and sharp mind, Levin is the longest-serving senator ever from Michigan. He was first elected to the Senate in 1978 by defeating a veteran Republican incumbent.
In 2002, with Republican President George W. Bush building toward an invasion of Iraq the following year, Levin was among those who opposed the war.
Levin voted against authorizing the use of force and unsuccessfully offered an alternative calling on Bush to pursue a tougher U.N. weapons inspection program in Iraq.
Bush's main justification for the war was to eliminate the threat posed by Iraq's weapons of mass destruction. No such weapons were found.
Levin has been among the chamber's most liberal senators and also among its biggest defenders of his state's auto industry.
Levin was born in Detroit on June 28, 1934. After Harvard Law School, he worked as a civil rights lawyer and public defender before being elected to the Detroit City Council in 1969. He later served as council president before running for the Senate.
His older brother Sander Levin has been a Democratic member of the U.S. House of Representatives since 1983.