  A dynamical analysis is presented that self-consistently takes into account
the motion of the critical layer, in which the magnetic field reconnects, to
describe how the m=n=1 resistive internal kink mode develops in the nonlinear
regime. The amplitude threshold marking the onset of strong nonlinearities due
to a balance between convective and mode coupling terms is identified. We
predict quantitatively the early nonlinear growth rate of the m=n=1 mode below
this threshold.
