It can be shown that for linearly unstable magnetohydrodynamic modes near the threshold of linear instability, the mode amplitude A (t) evolves according to ∂2A/Λt2 =λ2A+αA3 which can lead to nonlinear stabilization, explosive instability, or eventual decay, depending on the sign of α. This is demonstrated explicitly for the m=1 kink modes in a sharp boundary plasma pinch.