We demonstrate an electrically driven mechanism for controlling interfacial magnetic exchange and spin-polarized transport in a Co/Nb-doped HfO2/Co multiferroic tunnel junction. Using first-principles density functional theory and quantum transport calculations, we show that the ferroelectric polarization direction modulates the local orbital symmetry and hybridization pathways at the Nb–O–Hf interfaces, thereby switching the magnetic coupling between Nb and Hf from antiferromagnetic to ferromagnetic. In the antiparallel magnetic configuration, this orbital-selective control reverses the dominant spin channel and produces nearly complete, electrically switchable spin polarization in the zero-bias transmission over a broad energy window around the Fermi level. Our findings reveal the key role of polarization-driven orbital engineering in achieving deterministic, electrically tunable spin currents, providing a new pathway for designing high-performance spintronic devices.