Experimental results in the DIII-D tokamak demonstrate that discharges are more robust with regard to external nonaxisymmetric error fields as the plasma fluid rotation is increased, but more sensitive as beta is increased. In a series of discharges, the plasma fluid rotation and beta are varied by the number and type ('tangential' and/or 'perpendicular') of neutral beams. For each discharge, once rotation and plasma kinetic energy are equilibrated, an external coil is used to slowly increase the low m,n=1 resonant, static error field until a locked mode instability occurs. A larger critical error field is needed to nonlinearly induce instability in discharges spinning more rapidly. However, at higher beta, the plasmas become more unstable with increasing beta despite the faster rotation. The beta limit in tokamaks can be reduced by locked modes nonlinearly induced by modest error fields