  We present a first-principles approach to describe magnetic and
superconducting systems and the phenomena of competition between these
electronic effects. We develop a density functional theory: SpinSCDFT, by
extending the Hohenberg-Kohn theorem and constructing the non-interacting Kohn-
Sham system. An exchange-correlation functional for SpinSCDFT is derived from
the Sham Schl\"uter connection between the SpinSCDFT Kohn-Sham and a
self-energy in Eliashberg approximation. The reference Eliashberg equations for
superconductors in the presence of magnetism are also derived and discussed.
