Have you seen the ages of the lib SC Justices? I GUARANTEE YOU at least ONE of them won’t survive another four years. Ginsburg already has one foot in the grave.
The far more real threat is Scalia and Kennedy retiring because there is a GOP president. There is no way in the world Romney will appoint justices as conservative as these two.
Romney the liberal will appoint liberals just as he did in Massachusetts. The SC will then be solidly liberal for the next 20-30 years.
Its far preferable in my view to, at worst, let Obama appoint a liberal justice to replace Ginsburg than to let Romney decimate the conservative leaning court in the next eight years with the appointment of 3 or 4 liberal justices.