Think of his money, one Bloomberg aide told me, as powering a skunkworks for city innovations. It allowed him to reduce political risk, because he could foot the bill for policy experimentation himself, instead of leaving it to the taxpayers. When Bloomberg wanted to try covering paid-leave time for city workers, he didn’t go to the city council to approve new funding for a program. When his administration wanted to reduce the number of garbage trucks on the road by splitting them between general waste and recyclables, he could use private money to underwrite the new equipment.

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In short, the money Bloomberg spent in office helped him to be more popular and successful as mayor than he otherwise would have been. Much of this money can’t be tracked by the usual means of measuring funds in politics: campaign-finance disclosures.

Bloomberg is generally seen as one of the most effective mayors in the history of the city. How much of that perception is based on the money he spent on ad campaigns that promoted his anti-smoking and healthy-eating initiatives; or on flying state legislators to his home in Bermuda, where they golfed and often walked away with big checks for their reelection committees; or on funding education-reform groups that backed his efforts to seize control of the public schools from the state government? How much of what seemed to be his organic grassroots support can be traced, directly and indirectly, to outlays of cash? “I think every other politician would love to have the money to reinforce their message and to tell people that, you know, they’re great,” Los Angeles Mayor Eric Garcetti said to me recently about Bloomberg’s methods. (Garcetti has tried to copy them, albeit without the massive personal wealth to subsidize them. He has also said he’s open to endorsing Bloomberg’s presidential run.)

Bloomberg’s private spending “is what defined him as somebody people had to like and at least respect—because he doesn’t kiss up; he doesn’t tell them what they want to hear,” says Kathy Wylde, the president and CEO of the Partnership for New York City, which often served as a cheerleading group for Bloomberg while he was mayor. What made him “somebody that people would vote for and admire as mayor were his roles as a philanthropist and a selfless advocate on issues.”

For his 2020 presidential run, top staff members of the Bloomberg nonprofits promoting gun control, climate-change prevention, and city innovation have started working for the campaign. He has unapologetically hamstrung the huge news organization he owns, Bloomberg News, which has gone from publishing implicitly pro-Bloomberg 2020 editorials in the weeks before he made his run official to suspending the editorial board and having many of the same people who were writing them also move over to the campaign; the political reporters there worry about not being taken seriously anymore. Using the standards by which other candidates are judged, all of this would be considered campaign expenses or hidden costs. Bloomberg shrugs this off, saying that because he’s never taken any campaign contributions, he can’t be bought.