Consider a Hamiltonian action of a compact Lie group G on a compact symplectic manifold (M, ω) and let G be a subgroup of the diffeomorphism group Diff M . We develop techniques to decide when the maps on rational homotopy and rational homology induced by the classifying map BG → BG are injective. For example, we extend Reznikov's result for complex projective space CP n to show that both in this case and the case of generalized flag manifolds the natural map H * (BSU (n + 1)) → H * (BG) is injective, where G denotes the group of all diffeomorphisms that act trivially on cohomology. We also show that if λ is a Hamiltonian circle action that contracts in G := Ham(M, ω) then there is an associated nonzero element in π 3 (G) that deloops to a nonzero element of H 4 (BG). This result (as well as many others) extends to c-symplectic manifolds (M, a), ie, 2n-manifolds with a class a ∈ H 2 (M ) such that a n = 0. The proofs are based on calculations of certain characteristic classes and elementary homotopy theory.