Does one loss doom a politicians' career?
George Allen lost in 2006, and couldn't bounce back in 2012.
Democrats have lots of 2010 losers trying to get back into the Senate.
Not many former Senators have returned to the job since the 1950s.
As political analyst Stu Rothenberg put it, they're asking "voters to rehire them for a job from which they were fired." Senate Democrats are relying on these repeat candidates for the exact same reason that Democrats are comfortable with anointing Hillary Clinton for their presidential nomination: There aren't any better alternatives.
Hopefully for Republicans, the trend will continue in 2016.