  Axions and dark photons are common in many extensions of the Standard Model.
The dark axion portal -- an axion coupling to the dark photon and photon -- can
significantly modify their phenomenology. We study the cosmological constraints
on the dark axion portal from Cosmic Microwave Background (CMB) bounds on the
energy density of dark radiation, $\Delta N_\text{eff}$. By computing the
axion-photon-dark photon collision terms and solving the Boltzmann equations
including their effects, we find that light axions are generally more
constrained by $\Delta N_\text{eff}$ than from supernova cooling or collider
experiments. However, with dark photons at the MeV scale, a window of parameter
space is opened up above the supernova limits and below the experimental
exclusion, allowing for axion decay constants as low as $f_a \sim 10^4$ GeV.
This region also modifies indirectly the neutrino energy density, thus relaxing
the cosmological upper bound on the sum of neutrino masses. Future CMB
measurements could detect a signal or close this open window on the dark axion
portal.
