  A classical result of Erd\H{o}s and Rademacher (1955) indicates a
supersaturation phenomenon. It says that if $G$ is a graph on $n$ vertices with
at least $\lfloor {n^2}/{4} \rfloor +1$ edges, then $G$ contains at least
$\lfloor {n}/{2}\rfloor$ triangles. We prove a spectral version of
Erd\H{o}s--Rademacher's theorem. Moreover, Mubayi [Adv. Math. 225 (2010)]
extends the result of Erd\H{o}s and Rademacher from a triangle to any
color-critical graph. It is interesting to study the extension of Mubayi from a
spectral perspective. However, it is not apparent to measure the increment on
the spectral radius of a graph comparing to the traditional edge version
(Mubayi's result). In this paper, we provide a way to measure the increment on
the spectral radius of a graph and propose a spectral version on the counting
problems for color-critical graphs.
