  We calculate the broadband radio--X-ray spectra predicted by microblazar and
microquasar models for Ultra-Luminous X-ray sources (ULXs), exploring the
possibility that their dominant power-law component is produced by a
relativistic jet, even at near-Eddington mass accretion rates. We do this by
first constructing a generalized disk--jet theoretical framework in which some
fraction of the total accretion power Pa is efficiently removed from the
accretion disk by a magnetic torque responsible for jet formation. Thus, for
different black hole masses, mass accretion rates and magnetic coupling
strength, we self-consistently calculate the relative importance of the
modified disk spectrum, as well as the overall jet emission due to synchrotron
and Compton processes. In general, transferring accretion power to a jet makes
the disk fainter and cooler than a standard disk at the same mass accretion
rate; this may explain why the soft spectral component appears less prominent
than the dominant power-law component in most bright ULXs. We show that the
apparent X-ray luminosity and spectrum predicted by the microquasar model are
consistent with the observed properties of most ULXs. We predict that the radio
synchrotron jet emission is too faint to be detected at the typical threshold
of radio surveys to date. This is consistent with the high rate of
non-detections over detections in radio counterpart searches. Conversely, we
conclude that the observed radio emission found associated with a few ULXs
cannot be due to beamed synchrotron emission from a relativistic jet.
