  We present a unified theory of magnetism and superconductivity of $UGe_2$. To
this end, we consider part of $5f$ uranium electrons as mostly itinerant and
other ones as mostly localized. The main feature that distinguishes the
localized from the itinerant electrons is the effect of the pressure on them.
The pressure strongly screens the itinerant electrons while the localized ones
are almost unaffected. The screening of itinerant electrons leads to decreasing
of their Coulomb repulsion, therefore to formation of doubly occupied and empty
states. These states are spin-singlet and the effective spin of itinerant
electrons, the zero-temperature magnetization in units of Bohr magneton,
decreases. We obtain an effective two-spin Heisenberg model, which explains the
magnetization-temperature diagram of $UGe_2$. It is shown that the
experimentally observed characteristic temperature $T_x$, is a partial order
transition temperature. Below the Curie temperature $(T_x<T_C)$ the system
undergoes a transition from high temperature phase, were only localized
electrons contribute the magnetization, to the low temperature one, where both
itinerant and localized electrons contribute the magnetization. The
characteristic temperature decreases when pressure increases. At the quantum
partial order point $T_x=0$, the Zeeman splitting of the itinerant electrons is
zero. This permits formation of Cooper pairs and an onset of superconductivity
induced by the transversal fluctuations of the localized electrons. Small
deviation from the quantum partial ordered state leads to suppression of
superconductivity. This explains the dome form of the superconducting
transition temperature. The very low superconducting critical temperature is a
consequence of the Ising ferromagnetism.
