The Harvard Club of New York City, in Midtown Manhattan, is the embodiment of America’s old-money elite. Crimson-jacketed waiters serve members who are watched over by oil portraits of elite alumni.

One recent morning, financial advisers representing several dozen of America’s richest dynasties — advisers to the Pritzker and Soros families were listed as attendees — crowded into a drab meeting room on the club’s third floor.

The advisers were there to see Daniel Kowalski, a top aide to Treasury Secretary Steven Mnuchin and the Trump administration’s point person for the opportunity-zone rules. Mr. Kowalski is barnstorming the country, bouncing from one conference to the next, explaining to real estate investors and developers how to take advantage of the new rules.

Mr. Kowalski was an aide to the Trump campaign, where he worked for the White House policy adviser Stephen Miller. Before that, he was an aide to Jeff Sessions when Mr. Sessions was on the Senate Budget Committee.

[The Trump associates benefiting from a tax break for poor communities.]

At the Harvard Club, he dived into an explanation of how opportunity zones work — and for whom they work. “The audience for opportunity zones is inherently fairly small because it’s limited to capital-gains income, which is why I wanted to come and talk to this group,” he told the room of advisers.

That audience is small indeed: Only 7 percent of Americans report taxable capital gains, and nearly two-thirds of that income was reported by people with a total annual income of $1 million or more, according to I.R.S. data.

Yet this is a vital constituency, since the success of the opportunity-zone program will hinge largely on how much money investors kick in. That is why the Trump administration — and Mr. Kowalski in particular — is promoting the tax break on Wall Street.