  We study the onset of spin-density wave order in itinerant electron systems
via a two-dimensional lattice model amenable to numerically exact,
sign-problem-free determinantal quantum Monte Carlo simulations. The
finite-temperature phase diagram of the model reveals a dome-shaped $d$-wave
superconducting phase near the magnetic quantum phase transition. Above the
critical superconducting temperature, we observe an extended fluctuation
regime, which manifests itself in the opening of a gap in the electronic
density of states and an enhanced diamagnetic response. While charge density
wave fluctuations are moderately enhanced in the proximity of the magnetic
quantum phase transition, they remain short-ranged. The striking similarity of
our results to the phenomenology of many unconventional superconductors points
a way to a microscopic understanding of such strongly coupled systems in a
controlled manner.
