From Raman spectroscopy, magnetization, and thermal expansion on the system La-2/3(Ca1-xSrx)(1/3)MnO3, we have been able to provide a quantitative basis for the heterogeneous electronic model for manganites exhibiting colossal magnetoresistance (CMR). We construct a mean-field model that accounts quantitatively for the measured deviation of T-C(x) from the T-C predicted by de Gennes double-exchange in the adiabatic approximation and predicts the occurrence of a first-order transition for a strong coupling regime, in accordance with the experiments. The existence of a temperature interval T-C< T < T-*, where CMR may be found, is discussed in connection with the occurrence of an idealized Griffiths phase.