When the future first couple met in late 1949, Ronald Reagan was a broken man—literally. He showed up for their first date leaning on two canes, after having spent months in traction from an accident that shattered his thighbone in a half dozen places.

Spiritually, Reagan was in what he described as a “deep freeze.” He was approaching middle age and his movie career, never particularly illustrious, was hitting bottom. His first wife, Jane Wyman, had gotten bored with him and walked out, taking their two children with her. And he carried with him the emotional scar tissue of his precarious childhood as the son of an alcoholic.

“Then along came Nancy Davis,” he would later say, “and saved my soul.”

No one, including Reagan himself, could have imagined back then where his awakening interest in politics would lead the two of them. But looking back, it’s possible to understand how essential she was as a political partner, helping to build the scaffolding for her husband’s political rise and serving as his ever-vigilant protector in the White House and beyond.

Nancy Reagan exercised an influence unlike any first lady before or since. Hers was the power of intimacy. As affable as Ronald Reagan was, there was a remoteness to his nature. He was at heart a loner who liked people but, with the exception of his wife, didn’t need them. “He doesn’t let anybody get too close,” Nancy acknowledged. “There’s a wall around him. He lets me come closer than anyone else, but there are times when even I feel that barrier.”

So it fell to Nancy to be the networker of the two. In the early years of his political career, she was constantly on the phone with the rich benefactors who fueled her husband’s rise, stroking their egos and soliciting their opinions. “She cultivated them and maintained them in a way that my father wouldn’t have; wouldn’t have occurred to him, really,” their son, Ron Reagan, told me.

Nancy, wary by nature, was the shrewder judge of people. She recognized that, unless Ronald Reagan had the right set of advisers, he could be led astray by his trusting nature and tendency to delegate. She was also capable of doing what her conflict-averse husband would not—getting rid of those who did not put Ronald Reagan first.

When his 1980 campaign was on the verge of imploding, she engineered a needed shake-up in which the campaign manager and his top lieutenants were fired and a new leadership team brought in. While she rarely set foot in the West Wing during his presidency, her presence was felt by everyone who worked there. If she was displeased about something, they all knew it, and those who were not in her good graces tended not to last for long. A pragmatist, Nancy Reagan particularly mistrusted the ideologues who surrounded him—the types she described as “jump-off-the-cliff-with-the-flag-flying conservatives.”

“She was the guardian,” recalled James A. Baker III, who was the president’s first chief of staff and later his Treasury secretary. “She had a terrific political antenna, much better than his, in my view.”

Her instincts, time would show, were usually right—most crucially when his presidency was on the brink of collapsing under the Iran-Contra scandal during his second term. It was Nancy who remained clear-eyed enough to put together the rescue effort. She was relentless and ruthless in engineering the firing of Donald T. Regan, the autocratic White House chief of staff. She pushed her recalcitrant husband into acknowledging—to himself and the country—that he had made a massive blunder by trading arms to Iran in exchange for the release of American citizens who were being held hostage in the Middle East.

Nancy Reagan was determined that her husband go down in history as a figure of significance. Ending the Cold War, she believed, could be the accomplishment that secured Ronnie’s reputation as a giant among American presidents, and she pressured him constantly to move in that direction, often to the consternation of his more hawkish advisers. The Reagans quarreled over his hard-edged rhetoric, which included referring to the Soviet Union as an “evil empire.”

Her view was rooted not in geopolitical strategy but rather in her faith in her husband. Nancy Reagan “was more receptive to the idea of forming a working relationship with the Soviets than some of us were, and more willing to trust them,” said Caspar Weinberger, who served as defense secretary. “She believed strongly in his negotiating capabilities.”

As the years have gone by, appreciation has grown for the role that she played in assuring Ronald Reagan’s success and elevating him to a figure who would continue to shape politics for a generation after he left office.

Yet though she was hypervigilant in tending to her husband’s image, Nancy Reagan was clueless about managing her own. He was called the Teflon President because nothing bad ever seemed to stick to him. If that was the case, she was the Velcro First Lady. She was an easy proxy for political opponents who were too intimidated by Ronald Reagan’s popularity to attack him directly.

Terrified for her husband’s safety after he was nearly killed by a would-be assassin just two months after he took office, she turned to an astrologer to determine when and how he should travel and make public appearances. Her purchase of more than $200,000 worth of White House china created a headache for her husband amid a recession during which the Reagan administration was cutting poverty programs. She “borrowed” designer clothes and did not give them back.

The final, sad chapter of the Reagans’ lives together would bring a reassessment of Nancy. Even her most cynical critics were moved by the stoicism and devotion she showed during the last decade of her husband’s life, as he descended deeper and deeper into Alzheimer’s disease. In his final years and beyond, she became the shaper and guardian of his legacy, culminating in the publication of his presidential diaries and volumes of his letters.

For the acclaim and sympathy that finally came her way, she paid the highest price imaginable. Theirs had been a monumental story, and she was left to write the ending alone.

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By Karen Tumulty for The Wall Street Journal

—Ms. Tumulty is a political columnist for the Washington Post. This essay is adapted from her new book, “The Triumph of Nancy Reagan,” which will be published by Simon & Schuster on April 13.