The featured act at the Cellar Door these nights is Tom Rush, who roams -- wild and free -- a vast musical territory bounded on the east by Tom Paxton and on the west by Arlo Guthrie. A wise choice, because almost anyone else would make the evening anticlimactic with Casse Culver as the opening act.

Let's take care of Rush first -- a distinctive and able artist, mellow and flexible of voice, who can vary his style from "Duncan and Brady" to "Joshua Gone Barbados" at the drop of a chord and be completely idiommatic in either. The first is a rambling and very funny modified talking blues about a Western sheriff who busts folks for "havin' fun in the first degree"; the second, a song of social protest and personal desolation with the special lilt that the Caribbean gives even to its sad music. He is very good in both of these and at many points in between.

Culver's style is basically country, but the range is wide: mostly "the subject of love: L-U-S-T" in such wryly tender songs as "I'm Late Again," but with a fine satirical edge in her tribute to the "Bethesda Badlands." Culver opens again tonight; Rush is there through Wednesday. Go tonight.