Jefferson Davis has his day; Jefferson Davis Gets His Citizenship Back, By Robert Penn Warren. Lexington, Ky.: University Press of Kentucky. $8.75.

The restoration of citizenship to Confederate President Jefferson Davis in 1978, the event that occasioned this biographical essay by Robert Penn Warren, came about as an aftermath of an earlier reinstatement of Robert E. Lee in 1975. That event, coming in the wake of the Vietnam war, if Lee received his citizenship back, so should the draft evaders. In the end, Lee won out, championed by an unbeatable combination of Sens. Harry F. Byrd and Hubert H. Humphrey and President Gerald R. Ford.

Why, is view of Warren's unimpeachable record of civil libertarianism, did he choose to write this defense of an advocate of slavery and secession?

The answer seems to be the loyalty of one eminent son of Todd County, Ky., to another. Todd County staged a four-day celebration of Davis's reinstatement, and Warren was invited to give the main address.

As expanded in New Yorker magazine and reprinted in this book, it is a discourse not only on Davis, but on the persistence of the Confederate experience in the Southern consciousness.

When Warren was about 10, he heard that a monument was to be erected in nearby Fairview to Jefferson Davis, "whoever . . . he was." Warren became fascinated with Davis's "devotion to an objective, endeavor, the locale of which may be a poet's desk or a soldier's battlefield."

It's not so much what a man does, Mr. Warren came to believe, as the integrity with which he does it. Davis was a slaveholder who treated his wards with kindness, providing them with free time for personal profit, regular dental care, ceremonies of marriage and birth, and black jury trial. He was a secessionist who went to jail rather than ask pardon for what he steadfastly maintained was a constitutional right. "Honor, perhaps, more than victory," says Mr. Warren, "was, in the midst of ill fortune, ineptitudes, and even stupidities, his guiding star."

But is dedication enough? Before the war Davis was an inflexible foe of compromise on the issue of slavery. During the war he proved incapable of supplying the leadership to overcome the fatal states'-rights divisions within the Confederacy -- a contemporary marveled at his "facility for converting friends into enemies."

Mr. Warren deftly portrays him as withdrawn, self-righteous, and troubled by anxiety and illness. Even in family life he remained obdurate, destroying his daughter Winnie's chance for happiness by denying her permission to marry a promising Northern lawyer whose grandfather had been an abolitionist.

Still, in inspiring Todd County's festivities, Davis's reinstatement has had a happy result. It has given us this personalized testament to the image of the President of the Lost Cau se and in the process a superb evocation of the Southern mystique.