  Measurements of angular correlations in nuclear beta decay are important
tests of the Standard Model (SM). Among those, the so-called D correlation
parameter occupies a particular place because it is odd under time reversal,
and because the experimental sensitivity is at the $10^{-4}$ level, with plans
of further improvement in the near future. Using effective field theory~(EFT)
techniques, we reassess its potential to discover or constrain new physics
beyond the SM. We provide a comprehensive classification of CP-violating EFT
scenarios which generate a shift of the D parameter away from the SM
prediction. We show that, in each scenario, a shift larger than $10^{-5}$ is in
serious tension with the existing experimental data, where bounds coming from
electric dipole moments and LHC observables play a decisive role. The tension
can only be avoided by fine tuning of the parameters in the UV completion of
the EFT. We illustrate this using examples of leptoquark UV completions.
Finally, we comment on the possibility to probe CP-conserving new physics via
the D parameter.
