We study the effect of the disorder on the metallic behavior of a two-dimensional electron system in silicon. The temperature dependence of conductivity $sigma (T)$ was measured for different values of substrate bias, which changes both potential scattering and the concentration of disorder-induced local magnetic moments. We find that the latter has a much more profound effect on $dsigma/dT$. In fact, the data suggest that in the limit of $Tto 0$ the metallic behavior, as characterized by $dsigma/dT < 0$, is suppressed by an arbitrarily small amount of spin flip scattering by local magnetic moments.
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