In 1990, Dianne Feinstein retreated to a beach house on the coast of Northern California after she lost a grueling race to become the state’s governor. She was 57, and her political career appeared to have peaked at “mayor of San Francisco.” She told friends that she was thinking about leaving public life for good.

On the day after the election, former President Jimmy Carter sent her a handwritten note. “I’ve won some & lost some, so I can share some of your feelings,” wrote Mr. Carter, who had lost his bid for a second term in the White House 10 years earlier. That defeat at the time had seemed “a tragedy.” But since then, he went on, his life had been more full and productive than he could have ever imagined.

Two years later, Ms. Feinstein won a seat in the Senate, where she remained until she died at the age of 90 last year.

Her vacation refuge in Stinson Beach, Calif., has a new owner. Her official papers — some 5,000 boxes just from her years in the Senate — are at the Stanford University library archives. “Two serious groups are circling” the stately Washington, D.C., manor that she and her husband bought after her Senate victory, according to Ben Roth, the real estate agent handling the property.