  We consider a generalization of the contact process stochastic model,
including an additional autocatalitic process. The phase diagram of this model
in the proper two-parameter space displays a line of transitions between an
active and an absorbing phase which starts at the critical point of the contact
process and ends at the transition point of the voter model. Thus, a crossover
between the directed percolation and the compact percolation universality
classes is observed at this latter point. We study this crossover by a variety
of techniques. Using supercritical series expansions analyzed with partial
differential approximants, we obtain precise estimates of the crossover
behavior of the model. In particular, we find an estimate for the crossover
exponent $\phi=2.00 \pm 0.02$. We also show arguments that support the
conjecture $\phi=2$.

Statistical Mechanics