  We present a unified Boltzmann-transport theory for the drag resistivity in
two-component systems close to a second-order phase transition. We find general
expressions for the drag resistivity in two and three spatial dimensions, for
arbitrary population and mass imbalance, for particle- and hole-like bands, and
show how to incorporate, at the Gaussian level, the effect of fluctuations
close to a phase transition. We find that the proximity to the phase transition
enhances the drag resistivity upon approaching the critical temperature from
above, and we qualitatively derive the temperature dependence of this
enhancement for various cases. In addition, we present numerical results for
two concrete experimental systems: i) three-dimensional cold atomic Fermi gases
close to a Stoner transition and ii) two-dimensional spatially-separated
electron and hole systems in semiconductor double quantum wells.
