  C$_3$P$^-$ is analogous to the known interstellar anion C$_3$N$^-$ with
phosphorus replacing the nitrogen in a simple step down the periodic table. In
this work, it is shown that C$_3$P$^-$ is likely to possess a dipole-bound
excited state. It has been hypothesized and observationally supported that
dipole-bound excited states are an avenue through which anions could be formed
in the interstellar medium. Additionally, C$_3$P$^-$ has a valence excited
state that may lead to further stabilization of this molecule, and C$_3$P$^-$
has a larger dipole moment than neutral C$_3$P ($\sim 6$ D vs. $\sim 4$ D). As
such, C$_3$P$^-$ is probably a more detectable astromolecule than even its
corresponding neutral radical. Highly-accurate quantum chemical quartic force
fields are also applied to C$_3$P$^-$ and its singly $^{13}$C substituted
isotopologues in order to provide structures, vibrational frequencies, and
spectroscopic constants that may aid in its detection.
