  On the basis of general considerations, we propose a Langevin equation
accounting for critical phenomena occurring in the presence of two symmetric
absorbing states. We study its phase diagram by mean-field arguments and direct
numerical integration in physical dimensions. Our findings fully account for
and clarify the intricate picture known so far from the aggregation of partial
results obtained with microscopic models. We argue that the direct transition
from disorder to one of two absorbing states is best described as a
(generalized) voter critical point and show that it can be split into an Ising
and a directed percolation transitions in dimensions larger than one.
