"They were everything a growing nation needed for a symbol of success, and the country was not to see this combination of youth and daring again until the later cults of hero worship for George and Elizabeth Custer, Charles and Ann Lindbergh, or John and Jacqueline Kennedy," wrote the biographer Richard Egan about the subjects of Sally Denton's "Passion and Principle: John and Jessie Frémont, The Couple Whose Power, Politics, and Love Shaped Nineteenth-Century America."

John Frémont (1813–1890), called "The Pathfinder" for his repeated forays into the treacherous West at a time when California still belonged to Mexico and Britain still staked a claim on Oregon, was celebrated for intrepid journeys (surviving the hazardous Rockies, hostile Indians, and death-threatening diseases) that made him the embodiment of manifest destiny.

Jessie Frémont (1824–1902), the daughter of Missouri Senator Thomas Hart Benton, one of the most progressive and learned politicians this country has ever produced, sat at the knee of Andrew Jackson in the White House, where, in moments of stress, the President would unconsciously clench her hair. Taught not to complain, Jessie became an independent, spirited woman far ahead of her time, working with her husband on best-selling narratives of his adventures and managing his 1856 presidential campaign.

So how did it all go wrong? And how did this couple right themselves only to suffer enough repeated setbacks and comebacks to fill at least five seasons of an HBO series? The trouble began shortly after they met.

While Benton admired the young explorer and touted his prospects, it was quite another matter when Frémont fell in love with 15-year-old Jessie. Smitten with the 26-year-old Frémont, Jessie married John in a secret ceremony performed by a Catholic priest, apparently the only member of the clergy not sufficiently worried about Benton's wrath.

Eventually reconciled to the marriage, Benton again sponsored Frémont, who was promoted quickly to colonel in the U.S. Army, but who also aroused the envy of senior officers. They resented his popularity and tendency to take action without (so they thought) proper authority. What should have been Frémont's crowning glory, conquering California without war, turned into a court martial when he refused to cede command to President Polk's handpicked replacement. Frémont's original orders, Ms. Denton explains, were ambiguous, allowing Polk to retain or replace Frémont depending on the president's closely held political objectives.

For all Frémont's skills — he was a trained scientist, engineer, and cartographer — he had no political brain, and he never stooped to study politics. Charged with being a secret Catholic during the 1856 presidential campaign, he would not even issue a denial, let alone go on the offensive against his inept but ultimately victorious opponent, James Buchanan, a Democratic Party hack.

Jessie always had to do the heavy lifting, tirelessly trying to get her father to support her husband's presidential bid, for example. Benton was anti-slavery, pro-Western exploration, and so a natural Frémont ally, but Benton could not abide his son-in-law's high-handed moral tone or his inability to see that preserving the Union came first. Benton thought, and rightly so, that Frémont would make a terrible president, although Ms. Denton seems to demur on this point.

Jessie, a stellar player in her husband's campaign (she was the first presidential candidate's wife to make widespread public appearances), became the target of critics who decried such a visible role for a woman. She never wavered in her husband's support, even when advisers close to his campaign resigned, suspecting him of infidelity (rumors of his affairs would continue even after he abandoned politics).

Later she made a major blunder: In 1861, she went directly to President Lincoln to argue her husband's case—why it was necessary for Frémont (in charge of defending Missouri) to issue an Emancipation Proclamation before Lincoln was ready to countenance such a momentous act. Lincoln rejected her plea, even ridiculing her for arguing in her husband's stead.

Whether Frémont was morally right is beside the point. Ms. Denton calls his proclamation an act of courage and Jessie's plea a natural consequence of a woman at home in the White House. But Frémont's proclamation was also an act of political folly. No president can countenance an officer in the field announcing such a momentous policy on his own authority.

Disagreeing with Ms. Denton's judgments, however, is not as important as recognizing that she has written a riveting narrative about what she calls a "power couple" who "fascinated and baffled" the public. They are curiously modern and "evocative of Bill and Hillary Clinton," Ms. Denton rightly concludes. ( )