  The origin of the exceptionally strong superconductivity of cuprates remains
a subject of debate after more than two decades of investigation. Here we
follow a new lead: The onset temperature for superconductivity scales with the
strength of the anomalous normal-state scattering that makes the resistivity
linear in temperature. The same correlation between linear resistivity and Tc
is found in organic superconductors, for which pairing is known to come from
fluctuations of a nearby antiferromagnetic phase, and in pnictide
superconductors, for which an antiferromagnetic scenario is also likely. In the
cuprates, the question is whether the pseudogap phase plays the corresponding
role, with its fluctuations responsible for pairing and scattering. We review
recent studies that shed light on this phase - its boundary, its quantum
critical point, and its broken symmetries. The emerging picture is that of a
phase with spin-density-wave order and fluctuations, in broad analogy with
organic, pnictide, and heavy-fermion superconductors.
