Richard C. Blum, a financier and major donor to Democrats — above all his wife, Senator Dianne Feinstein of California — died on Sunday at the family home in San Francisco. He was 86.

The cause was cancer, according to a statement by Ms. Feinstein’s office.

Ms. Feinstein missed votes in recent weeks as Mr. Blum’s health declined, imperiling the Democrats’ precarious Senate majority, which relies on a tiebreaking vote from Vice President Kamala Harris.

Mr. Blum exerted influence generally as a political patron and an adviser. It was only during his final months that he sought government office, informing President Biden early last year that he hoped to be made an ambassador.

That move could have caused Ms. Feinstein to leave the Senate and travel overseas with Mr. Blum, and some Democrats saw it as a potential way for her to make a graceful exit from Congress at age 87. Members of her own Democratic caucus had been grumbling that her mental acuity had diminished and that she had become too accommodating to Republicans.

Mr. Blum became Ms. Feinstein’s companion between the death of her second husband, Bertram Feinstein, in 1978, and her victory in a race for mayor of San Francisco in 1979 after two unsuccessful attempts. They married in 1980. In 1983, Mr. Blum helped Ms. Feinstein raise $400,000 to beat back a recall attempt.

He remained her closest confidant and most reliable fund-raiser through her mayoralty, which ended in 1988. The couple then poured about $3 million of their own money into the 1990 California governor’s race, which Ms. Feinstein lost to Pete Wilson, a Republican. But, again with Mr. Blum’s financial help, she won election to the Senate in 1992.

Ms. Feinstein is now the fifth-longest-serving United States senator, and if she stays in office for just under a year longer she will surpass Barbara A. Mikulski as the longest-tenured female senator in American history.

Mr. Blum ran his own investment firm, Blum Capital Partners, and during his career as an investment banker and financial manager his clients included large institutions like Bank of America.

With deep pockets, he became a major figure in Democratic politics, counting Jimmy Carter as a jogging partner and Mr. Biden as one of many beneficiaries of his largess. His net worth was estimated to exceed $1 billion.

He also held nonprofit positions; at one point he was chairman of the University of California Board of Regents.

With the Dalai Lama among his influential friends, Mr. Blum developed an interest in South Asia as the home of Buddhist philosophy, as a recipient of philanthropy and as a place for adventure: He once led an expedition up part of Mount Everest.

In a statement of condolence, Mr. Biden called Mr. Blum “a successful businessman and proud son of California who dedicated much of his public life to fighting poverty around the globe” through the establishment of the American Himalayan Foundation, a nonprofit group that builds schools and hospitals in Tibet, and the Blum Center for Developing Economies at the University of California, Berkeley, which focuses on innovative solutions to global poverty.

Richard Charles Blum was born on July 31, 1935, in San Francisco to Louise Hirsch and Herbert Blum, a seller of robes and raincoats who died during Richard’s boyhood. Richard graduated from Berkeley with bachelor’s and master’s degrees in business administration.

He joined Sutro & Company, a San Francisco brokerage firm, at 23 and became a partner before he turned 30, by which time he was already a millionaire.

In addition to Ms. Feinstein, Mr. Blum is survived by a brother, Robert; his daughters, Annette, Heidi and Eileen; his stepdaughter, Katherine; and seven grandchildren.

During Ms. Feinstein’s 1990 race to be California’s governor, Mr. Blum described to The New York Times what he called “the triathlon of politics.”

“No. 1,” he said, “we get to see on a regular basis everything she’s ever done and I’ve ever done distorted in the newspapers. No. 2, we get to share 17 years of our tax returns on an intimate basis with 30 million people. And three, I get to pay to watch all this happen.”

Shawn Hubler contributed reporting, and Kitty Bennett contributed research.