This paper is devoted to the theoretical analysis of nonideality effects in stretched rubber samples. The joint effect of packing forces and topological constraints is accounted for by adopting harmonic potentials between all pairs of network atoms, in addition to fixing a suitable set of junctions at the macroscopic sample surface according to the James-Guth theory. The potential minima are set at the average, affinely deformed interatomic distances. The force constant of each interaction is proportional to the probability of interatomic contact in the undeformed state and is inversely proportional to the square strain ratio along any space direction, thus accounting for the variation of the entanglement concentration with sample stretching. The proportionality factor of the pair potential is an adjustable parameter of the theory. A periodic coarse-grained model is used and the sample free energy is evaluated through normal-mode self-consistent analysis. Both the Mooney effect and the observed radius of gyration of the chain strands projected along different directions are properly accounted for. The results are similar to those of the Ronca-Allegra theory, which is based on direct application of constraints to the junction fluctuations. However, the present approach also embodies features of the theories which adopt the tube model. Finally, the variation of the Mooney constant C-2 With sample swelling is accounted for in a semiquantitative way.