Thomson ReutersMarch 7, 2013 19:45
* 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.