  We consider co--rotational wave maps from (3+1) Minkowski space into the
three--sphere. This is an energy supercritical model which is known to exhibit
finite time blow up via self-similar solutions. The ground state self--similar
solution $f_0$ is known in closed form and based on numerics, it is supposed to
describe the generic blow up behavior of the system. We prove that the blow up
via $f_0$ is stable under the assumption that $f_0$ does not have unstable
modes. This condition is equivalent to a spectral assumption for a linear
second order ordinary differential operator. In other words, we reduce the
problem of stable blow up to a linear ODE spectral problem. Although we are
unable, at the moment, to verify the mode stability of $f_0$ rigorously, it is
known that possible unstable eigenvalues are confined to a certain compact
region in the complex plane. As a consequence, highly reliable numerical
techniques can be applied and all available results strongly suggest the
nonexistence of unstable modes, i.e., the assumed mode stability of $f_0$.
