The metal-insulator (MI) transition in epitaxial thin films of La0.75 Ca0.25 MnO3 (LCMO) is accompanied by the appearance of an intrinsic electrical nonlinearity. The latter, probed by electrical third harmonic voltage, U3ω, or resistance, R3ω =d U3ω /dJ, is drastically enhanced in the vicinity of the MI transition, TMI =267 K. Applied magnetic field, B=5 T, suppresses the nonlinearity, resulting in a huge "nonlinear" CMR3ω (TMI) ∼ 105 %. R3ω shows a peculiar low-frequency (ω≤1 kHz) dependence, R3ω ∼ (ω- ω0) n, with exponent, n, changing across the MI transition from n∼1,5-2 for T< TMI to n=1 (T< TMI). The observed electrical nonlinearity in LCMO reflects the behavior of correlated polarons, the number of which dramatically enhances in the vicinity of TMI. We argued that correlated polarons, considered as electric-elastic quadrupoles, provide a nonlinear (quadratic) coupling to the electric field, yielding a third harmonic electric nonlinearity in LCMO. The reference film of La0.7 Sr0.3 MnO3 (LSMO), a prototypic double exchange system with second-order phase transition, is characterized as a linear metallic material in the whole range of temperatures (T=10-400 K), magnetic fields (B=0-5 T), and frequencies (ω=1-1000 Hz).