  Diverse astrophysical observations suggest the existence of cold dark matter
that interacts only gravitationally with radiation and ordinary baryonic
matter. Any nonzero coupling between dark matter and baryons would provide a
significant step towards understanding the particle nature of dark matter.
Measurements of the cosmic microwave background (CMB) provide constraints on
such a coupling that complement laboratory searches. In this work we place
upper limits on a variety of models for dark matter elastic scattering with
protons and electrons by combining large-scale CMB data from the Planck
satellite with small-scale information from Atacama Cosmology Telescope (ACT)
DR4 data. In the case of velocity-independent scattering, we obtain bounds on
the interaction cross section for protons that are 40\% tighter than previous
constraints from the CMB anisotropy. For some models with velocity-dependent
scattering we find best-fitting cross sections with a 2$\sigma$ deviation from
zero, but these scattering models are not statistically preferred over
$\Lambda$CDM in terms of model selection.
