THOUGH REPUBLICANS and many political handicappers in Virginia claim that the campaign for a U.S. Senate seat in the state is over already, the voters have yet to confirm the big edge now being given incumbent John W. Warner over Democratic challenger Edythe C. Harrison. We have concluded that Mr. Warner deserves re- election, but we do so with a higher regard for the candidacy of Mrs. Harrison than the polls and the state leadership of her own party seem to show. Mrs. Harrison has campaigned with vigor and raised some interesting points, and she is not the dilettante or political flake those politicians who resent her style would have people believe. Her campaign suggests that she may be a continuing presence in Virginia politics.

Nevertheless, Mrs. Harrison has not demonstrated -- to us, anyway -- that she would be a better or more effective senator from Virginia than Mr. Warner. Readers who recall our assessment of Mr. Warner in his original election effort six years ago will know that this is a cataclysmic change of view for us.

Not that we regard the incumbent as an inspirational voice in American politics; but on national as well as regional affairs, Sen. Warner has served a useful role. He has functioned as part of one of the few governing entities that have acted creditably and with some special insight and courage over the past few years: the moderate Republican leadership and its political followers in the Senate. Generally this group has done a good job of checking and balancing the administration, and Mr. Warner -- who to his credit has parted on a number of votes from his GOP colleague from Virginia, Sen. Paul S. Trible Jr. -- has been a responsible team player on it. Mr. Warner has been attentive, too, to the interests of Northern Virginians, working cooperatively wih House and Senate colleagues from this region on issues of common concern such as Metro and National Airport.

Many of Mrs. Harrison's challenges have been constructive. She has raised pertinent questions about Sen. Warner's ties to defense interests and how they affect his growing role as a member and possibly the next chairman of the Armed Services Committee. Also, Mrs. Harrison has attempted to reach out to black voters and others who have yet to be absorbed in the mainstream of statewide politics, even if they have no great quarrel with Mr. Warner's own voting record on civil rights issues.

Mrs. Harrison is more impressive now than when she began her campaign, but then so is Mr. Warner more impressive in office than he was when running for it. If, as we believe it may, the challenge of Mrs. Harrison serves to broaden political debate as well as the nature of representation of the Old Dominion on Capitol Hill, this seemingly one-sided contest wll have more significance than it seemed it would at the outset.