  The effect of pressure on the B--T phase diagram of
$\alpha$-(BEDT-TTF)$_2$KHg(SCN)$_4$ is studied. The measured phase lines can be
well described by a recent model of a charge-density wave system with varying
nesting conditions. A remarkable increase of the transition temperature with
magnetic field is found in a certain pressure and field range. We associate
this result with a dramatic enhancement of the orbital effect of magnetic field
due to a deterioration of the nesting conditions by pressure. Furthermore, we
present data which can be interpreted as a first sign of field-induced
charge-density waves.

Strongly Correlated Electrons