William Devane is no stranger to political role-playing; he was John Fitzgerald Kennedy in The Missiles of October. Now he's a Joseph Kennedy-ish clan leader in THE MONROES (ABC, Sept. 12, 10-11 p.m.; Thursdays, 9-10 p.m. thereafter), a new prime-time drama series. An on-screen title tells us we're in ''Maryland 1995,'' and Devane's John Monroe is announcing a run for the governorship. Another title tells us we're going inside ''Monroe Mansion,'' and the soap opera begins.
Monroe's passel of grown children (among them, L.A. Law's Cecil Hoffmann) are involved in daddy's political life to varying degrees. Hoffmann's character is not only cold-blooded and intelligent; she's also sleeping with the President. John's wife, Kathryn, played with icy devilishness by Susan Sullivan, is both smart and long-suffering. And, of course, it doesn't take long for a mistress of John's to pop up and utter the line ''Your husband has an enormous appetite for conquest of all kinds.'' Most of The Monroes is as stiff as that snippet of dialogue, but Devane and Sullivan have obvious fun as would-be power brokers, and anyone who wishes Dynasty were still on may find this potboiler amusing. C