In his book “Franklin & Lucy: President Roosevelt, Mrs. Rutherfurd and the Other Remarkable Women in His Life,” which comes out next week, Joseph E. Persico suggests that the affair between Franklin Roosevelt and Lucy Mercer (later Rutherfurd), which was supposed to have ended in 1918, resumed even sooner than most scholars believed — indeed, that it may never have ended at all.

In an era of well-publicized “bimbo eruptions” and gubernatorial call-girl visits, the main lesson here is one we’ve been taught before: that in the old days politicians — with the help of an obliging press and retinues of discreet enablers — did a better job of keeping their private lives private. They faithfully observed what Gladstone, complaining about the morals of other prime ministers, called the 11th Commandment: Thou Shalt Not Get Caught.

Nor were we necessarily worse off back then for not knowing — not even suspecting — the kinds of things we think we know now. The Mercer relationship did not materially alter the Roosevelt presidency. The person it changed was Eleanor Roosevelt. But in the mostly grubby history of presidential liaisons, the persistence of the Roosevelt-Mercer connection suggests that this one was a genuine love affair — a meeting of soul mates, perhaps. For once, everyone behaved pretty well. And not just the guilty parties, but even the betrayed spouse gained a little something. In this last respect the Roosevelts may even remind us a bit of a more recent White House marriage.

In outline at least, the Mercer story is pretty straightforward. In 1913, on the advice of Anna Roosevelt Cowles, a family elder known as Aunty Bye, Eleanor Roosevelt hired Lucy Mercer, seven years younger, to be her social secretary. Her husband was newly installed as the assistant secretary of the Navy; she was in the early stage of pregnancy and overwhelmed with the demands of Washington society. Attractive and personable, Lucy quickly proved herself so useful and efficient that she became an ancillary member of the family.

She and Franklin probably became intimate in 1916, and the affair was discovered in September 1918, when Eleanor, unpacking for her husband, who had just returned from England with the flu, discovered a bundle of incriminating letters.

Eleanor offered Franklin a divorce, but Sara Delano, his formidable mother, stepped in and said that if he left his wife she would cut him off without a cent. Louis Howe, Franklin’s trusted adviser, said that a divorce would mean the end of his political career. So Franklin agreed to stay in the marriage under two conditions set down by Eleanor: he had to break off with Lucy Mercer immediately and for good, and he could never again share his wife’s bed.

Franklin observed the second part of the agreement. How long he kept the first has been a matter of some scholarly debate. Some believe that Lucy attended Roosevelt’s first inauguration, in March 1933, hiding in the back of a limousine he had sent for her. The White House logs show that someone named “Mrs. Paul Johnson” — believed to be a pseudonym for Lucy Rutherfurd as she was then, having married Winthrop Rutherfurd, a wealthy widower — visited in August 1941 and again in November. And it has been known for some time that after Winthrop’s death in 1944, Lucy began seeing Franklin fairly regularly, and that she, and not his wife, was with him when he died in Warm Springs, Ga., in April 1945.

While working on his book, Mr. Persico obtained from Lucy Mercer’s granddaughters a bound copy of a lecture Roosevelt gave at Milton Academy in May 1926, on the flyleaf of which is an inscription saying, “I dedicate this little work, my first, to you.” The granddaughters also gave Mr. Persico some letters that Franklin wrote to Lucy beginning in 1926 on the letterhead of the Fidelity and Deposit Company of Maryland, where he was then a vice president. The letters are chatty rather than romantic, but as Mr. Persico points out, they’re also unusually specific about his whereabouts at certain times and also about Eleanor’s absences. Some historians have suggested that all through the 1920s and ’30s, Roosevelt was on such a tight leash, guarded by his secretary, the vigilant Missy LeHand, that he couldn’t possibly have strayed. But Mr. Persico reads these letters as possible plans for liaisons.

As for Eleanor Roosevelt, Mr. Persico leaves little doubt not only that she was devastated by the discovery of the affair but that she continued to love her husband to the end. She was crushed all over again to learn that Lucy Mercer had returned to Franklin’s life, and it took her a while to forgive her daughter, Anna, who had engineered some of their meetings.

But Mr. Persico, like other biographers, also suggests that in a way the affair was the making of Eleanor Roosevelt. She started standing up to her tyrannical mother-in-law. She cultivated important friendships of her own, most notably with Earl Miller, her driver and bodyguard, and with the journalist Lorena Hickok. And, always drawn to causes, like helping the downtrodden, she began fashioning herself into the person now remembered and revered — the champion of the New Deal, the tireless advocate of civil rights. She set such a powerful example that, as her biographer Blanche Wiesen Cook pointed out in a recent interview, Lucy Mercer began to model herself after Eleanor, taking political interest in a way she hadn’t before and ardently supporting integration.

Eleanor became, in brief, a sort of Hillary — a presidential spouse with a life, a mind and political aspirations and agendas of her own. She wrote later: “I had gained some assurance about my ability to run things and the knowledge that there is joy in accomplishing a good job. I knew more about the human heart.”

Who knows what goes on in anyone’s bedroom, let alone the presidential one? Mr. Persico gives us a Franklin Roosevelt of almost Clintonian appetite, and suggests that he may also have had affairs with Missy LeHand and with Daisy Suckley, a cousin who was also there when he died. But the evidence suggests that his real idea of pleasure at the end of the day was just to smoke a cigarette, have a couple of drinks and bask in the attention of some admiring females. That we want to read sex into this probably says more about us than about him.

Lucy Page Mercer Rutherfurd