Even opponents of the Reagan presidency and Nancy Reagan’s performance as First Lady might have shed a tear as she bid farewell to her beloved “Ronnie” at his 2004 burial at the Ronald Reagan Presidential Library in Simi Valley, Calif. Concluding the perfectly planned service, with the sun setting behind picturesque mountains, as often seen at the end of Hollywood westerns, Nancy leaned over the former president’s casket, wept and choked out the words, “I can’t leave him.”

It marked the first time, after their 52-year marriage, that she couldn’t control her access to him. In The Triumph of Nancy Reagan, her deeply researched and compellingly crafted biography of the 40th president’s second wife, Washington Post columnist Karen Tumulty paints a striking portrait of how this unique partnership shaped Ronald Reagan’s political career, from the California governor’s mansion to the White House.

[T]hese partnerships . . . frequently shape the paths of politics and policy.

A historian recently told me she couldn’t care less about presidential marriages. But we should, not only because these partnerships can be fascinating windows into presidential character but also because they frequently shape the paths of politics and policy. How so and how much are perennial questions for First Lady biographers and scholars. Because presidential spouses serve in unelected, unofficial roles, the American people draw strict limits around their perceived influence. Stray too far beyond their parameters, as Eleanor Roosevelt and Hillary Clinton were thought to have done by their adversaries, and a backlash can result.

As she entered the White House in 1981, Nancy Reagan rarely emphasized public policy as a primary concern. With her celebrity patina and designer fashion tastes, she resembled Jacqueline Kennedy more than feminists Betty Ford or Rosalynn Carter. Nancy’s unyielding and adoring gaze focused solely on her husband. In each other, the Reagans discovered the missing elements of their difficult childhoods. Alcoholism plagued Ronnie’s father and created an unstable family that moved frequently as his job prospects deteriorated. The future president adored his pious mother, who encouraged his early stage performances.