We study applicability conditions of the Nehari manifold method for the equation of the form $ D_u T(u)-λD_u F(u)=0 $ in a Banach space $W$, where $λ$ is a real parameter. Our study is based on the development of the theory Rayleigh's quotient for nonlinear problems. It turns out that the extreme values of parameter $λ$ for the Nehari manifold method can be found through the critical values of a corresponding nonlinear generalized Rayleigh's quotient. In the main part of the paper, we provide some general results on this relationship. Applications are given to several types of nonlinear elliptic equations and systems of equations.