WASHINGTON (AP) - Democratic Sen. Carl Levin's decision to not seek re-election in 2014 has set the stage for a wide-open race to replace the longest-serving senator in Michigan history. First elected to the Senate in 1978, the 78-year-old Levin announced Thursday that he has decided to focus on serving as Senate Armed Services chairman and an advocate for his home state without the distraction of campaigning. 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 and the power in the rest of state government. Democrats, who control 55 seats in the U.S. 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. The last time Michigan had an open Senate seat was in 1994, and Levin's retirement could draw a large field of potential successors. Rep. Gary Peters, a Democrat who represents suburban Detroit, has been viewed as a potential candidate along with former Rep. Mark Schauer and Democratic National Committee member Debbie Dingell, the wife of Rep. John Dingell. Potential GOP candidates include Reps. Mike Rogers, Dave Camp, Candice Miller and Justin Amash and Lt. Gov. Brian Calley. Attorney General Bill Schuette said in a statement that he would not seek the Senate seat. Levin had won re-election handily in recent years and was considered a safe bet to hold onto the seat if he had sought another term. Levin's departure puts Democrats at a disadvantage as they already are looking for someone to challenge Gov. Rick Snyder. On the other hand, Democrats have fared well in federal elections in a state that has gone for Democrats in six straight presidential races. Just one Republican has won a Michigan Senate seat in 40 years, Spencer Abraham in 1994, a non-presidential year. "This is going to get very interesting quickly," said John Truscott, a media consultant who has worked on GOP campaigns. President Barack Obama called Levin a "true champion" for members of the military and 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." Former Gov. Jennifer Granholm, a Democrat, said in a statement that "to categorize his decision as 'a loss' seems a sweeping understatement." Snyder said Levin has been a "thoughtful, compassionate voice in Washington." Levin also has been an outspoken advocate for his home state auto industry and manufacturers. He attended Central High School in Detroit and spent time as a taxi driver and an auto factory worker, years later still carrying in his wallet a fading 1953 United Auto Workers membership card. He earned a bachelor's degree in political science from Swarthmore College in 1956 and a law degree from Harvard in 1959. In 1964, he was named an assistant state attorney general and the first general counsel for the Michigan Civil Rights Commission. Detroit voters in 1969 elected Levin to the first of two four-year terms on the City Council, known then as the Common Council. He served as council president in his second term, then ousted Republican Robert Griffin in the 1978 Senate election. Levin is famous for wearing his eyeglasses down on his nose. Associated Press writers Donna Cassata in Washington and David Eggert in Lansing, Mich., contributed to this report.