Democrats are facing challenging terrain as they try to hold onto their Senate majority in 2014 and prevent Republicans from gaining full control of Congress in President Barack Obama's final two years in office.
Democrats must defend 21 seats, including seven in largely rural states that Obama lost last fall. And four Democrats have announced plans to retire. The latest was Carl Levin of Michigan. A fifth Democratic retirement could come soon from South Dakota Sen. Tim Johnson.
Democrats control 55 seats in the Senate, meaning Republicans would need to pick up six seats next year to recapture the majority for the first time since 2006.
Republicans are eyeing several races in states carried last year by Republican presidential nominee Mitt Romney.