If Obama nominates an extremist, the Republicans can block the nomination with no harm. That assumes the Republicans find a better strategist and spokesperson than McConnell.
If Obama sees the trap he can set and nominates an alleged moderate Republican, or a Jane Harmon/Evan Bayh type moderate then it will be much harder for the Republicans to oppose that nominee.
My advice: Speak softly. Carry a big stick. But keep the stick concealed if possible. Dont go around loud-mouthing.
How do we get rid of lifetime appointments ?