Michael R. Bloomberg, the former mayor of New York City, under extraordinary pressure from friends and fellow billionaires to do more to help Vice President Kamala Harris, recently donated about $50 million to a nonprofit organization that is supporting her presidential run, according to four people with knowledge of the matter.

The donation followed months of arm-twisting from associates such as Bill Gates, investor Ron Conway and Reid Hoffman, the co-founder of LinkedIn. Mr. Bloomberg recently spoke with Ms. Harris in a private phone call, according to two people briefed on the conversation.

Mr. Bloomberg’s decision conforms to a strategy that has become his trademark: Confounding Democratic operatives by refusing to make early investments — only to come in hot and heavy in the homestretch. But unlike his previous big gifts, this one was intended to be kept under wraps, and that secrecy has made unaware Democrats more anxious than they have been most autumns.

His contribution was made to Future Forward USA Action, the dark-money vehicle of Future Forward, Ms. Harris’s main super PAC.

Mr. Bloomberg, who is 82 years old and has an estimated net worth of $105 billion, is the second largest disclosed individual donor to Democrats in this election cycle, after the investor and philanthropist George Soros. But publicly, Democrats observed that Mr. Bloomberg was donating nowhere close to what he had spent during the presidency of Donald J. Trump.

The $47 million he had given in federally disclosed political contributions during this election cycle, before his new nonprofit donation, was less than half of the $95 million he disclosed to help Democrats retake Congress in the 2018 midterms.