A perturbation scheme is constructed to describe the evolution of stable, localized
Rankine-type hydrodynamic vortices under the action of disturbances such as density
stratification. It is based on the elimination of singularities in perturbations by using
the necessary orthogonality conditions which determine the vortex motion. Along with
the discrete-spectrum modes of the linearized problem which can be kept finite by
imposing the orthogonality conditions, the continuous-spectrum perturbations play
a crucial role. It is shown that in a stratified fluid, a single (monopole) vortex can
be destroyed due to the latter modes before it drifts very far, whereas a vortex pair
preserves its stability for a longer time. The motion of the latter is studied in two
cases: smooth stratification and a density jump. For the motion of a pair under a
small angle to the interface, a complete description is given in the framework of our
theory, including the effect of reflection of the pair from a region with slightly larger
density.