HAVING BROUGHT to the stage such liberal characters as Clarence Darrow, Henry Thoreau and Auntie Mame, how will playwrights Jerome Lawrence and Robert E. Lee treat a conservative female lawyer from California's conservative Orange County?

The lady Ruth Loomis is not yet a public figure. Loomis is the name the playwrights have given to the first female Justice of the Supreme Court in their new comedy-drama, "First Monday in October," beginning previews Wednesday as the Eisenhower with Jane Alexander as the lady and Henry Fonda as the old liberal who is disgruntled by her appointment.

"Inherit the Wind," which put Lawrence and Lee on the dramatic map, was clearly titled in favor of liberal Darrow at the expense of William Jennings Bryan in their thinly disguised drama about the Scopes trial.

An overlooked footnote to Ohio's Kent State University history is the fact that "The Night Thoreau Spent in Jail" was being performed there when violence erupted on campus. Their portrait of New England's 19th-century free-thinker fit precisely the mood of student protests against Vietnam.

Until now their most sharply defined conservative figures have been Mr. and Mrs. Claude Upson, of Montebank, Westchester County, first in "Auntie Mame," then in its musical offspring, "Mame." They described the pair thus:

"She is a rosy, flaccid woman who thinks Walter Lippmann makes tea. Mr. Upson is loud and square . . . and thinks Walter Lippmann is a Socialist."

So - will Ruth Loomis be a caricature of all conservatives hold precious?

"Not at all," answers Lawrence. "We've made as convincing a case for our conservative ustice as we can. We do some of our playwriting by phone, Bob in his home in Encino, me in mine at Malibu. If anyone had been eavesdropping thinking to hear radical talk, we had them confused whenever we took, as both of us did, Ruth's party line.

"We've created two solid cases for our conservative and liberal to argue about seriously. Mahoney vs. Ohio is rooted in personal freedom and Abbott vs. Omni Tech is concerned with a multinational corporation's role in the energy crisis."

Lee continues: "People ask where we got the title. "That's not in the Constitution,' they say. No, it's not. 'First Monday in October' is a phrase from the Judiciary Act of 1987."

Before they get written, plays are mulled over. That's how "First Monday in October" began over four years ago. Thinking about the then - and still - current topic of a woman appointee to the Court, they asked each other: "What sort of woman would she be?" Since it then was a Republican administration, they decided that to make the appointment convincing she would be a Republican. Then the question became "How much, what kind, of a Republican?" They narrowed that to a philosophical "conservative," and the kernel of the play took shape. They are pleased that so far they've beaten all the Presidents to what they consider a noble action.

Next, Lawrence and Lee checked the records and realized that never had the Supreme Court been the setting for a play, though it's very much felt in "The Magnificent Yankee," Emmet Lavery's biographical play about Justice Oliver Wendell Holmes. Their mulling, then, was novel as well as topical.

One ingredient all playwrights have in common is lawyers, and soon Lawrence and Lee were prowling California, New York and Washington talking to their lawyers and, through them, to those who had dealings at the white marble building on Capitol Hill.

Their most generous source proved to be former Chief Justice Earl Warren, who spent hours with them shortly before his death. Carefully protecting any direct quotes to avoid making Warren seem like a gossip, Lawrence and Lee allow that Warren Confided to them some of the flavor of meetings known only to Justices. "His generosity and humorous confidences were a terrific boost to us," says Lee. "He confirmed our hunch that the Nine Old Men are intensely human people, not just cold, philosophical masks."

From their nosing around and imaginings came the play's first version. Then followed two rewrites and, two years ago, a production at the Cleveland Play House, with Jean Arthur and Melvyn Douglas in the Alexander-Fonda roles.

"Experience had taught us," remarks Lawrence, "that in today's theater don't set your cap immediately for New York. We learned from the Cleveland production what the basic flaws were. We eliminated one major character completely, throwing out an area of discussion which didn't belong to the particular play we were trying to write. That's the way it always seems to go, eliminate, refine, polish."

"Even now," continues Lee, "we're not heading directly for New York. This is being mounted solely for the Kennedy Center by Plumstead Productions, the producing group that's been active under Martha Scott and Fonda."

"It was Martha," says Lawrence, "who came through for us. Several producers were lukewarm. But as soon as she'd read it, at Fonda's suggestion, Martha started the production rolling. Ned Sherin became interested in directing it and got Jane, his wife, enthused over Ruth Loomis. So it's all fallen into place. But right after it closes here in February, Hank has some filming to do, so does Jane and we'll see how it has played in Washington. If it works here, maybe all will come together again next fall for a New York production.

"That's the way it works now," interjects Lee, and it's a lot more sensible than in the old days when everything was headed for New York. Every producer there turned down 'Inherit the Wind' but when, five years later, Margo (Jones) read it, she didn't hesitate about a production in her Dallas theater, where her rule was 'originals only.' She didn't mind that a cast of 65 would be playing to an audence of only a few hundred. Without her, 'Inherit the Wind' never would have gotten on a stage or a screen."

It was to salute the late Margo Jones that Lawrence and Lee not together with two other of her playwright discoveries, Tennessee Williams and William Inge, to create the annual Margo Jones Awards, given to theaters and organizations that stage new scripts, generally by unknowns. These have gone to theaters across the country, include one to Arena Stage, another to the defunct Washington Theater Club.

Lawrence and Lee also sparked the American Playwrights' Theater Club.

Lawrence and Lee also sparked the American Playwrights' Theater, an alliance of regional, university and community theaters that chooses one new script a year for its member organizations. This guarantees to the chosen playwright royalties from 50 to several hundred theaters, garnering the fiscal equivalent of a New York season's run.

A few years back Lawrence and Lee were two of about a dozen playwrights earning their livings in the American theater. Thanks to their Jones awards, APT and university teaching, they've increased the number of their competitors. Now scores of playwrights are earning livelihoods from their plays, not from advertising jingles.

"The more there are, the livelier our theater," says Lee.

"Call it what Ruth Loomis might," says Lawrence, "activist conservatism."