Bill de Blasio has made New York’s “Tale of Two Cities” a centerpiece of his Democratic campaign for mayor. “One New Yorker is rushing past an attended desk in the lobby of a majestic skyscraper,” Mr. de Blasio said in a speech at the New School earlier this year, while “a few miles away, a single mother is also rushing, holding her two young children by the hands as they hurry down the steps of the subway entrance.”

But Mr. de Blasio’s point was only partly correct. Relatively few wealthy owners of ultraluxury apartments are rushing through their buildings’ lobbies, because they’re rarely, if ever, there.

To address the inequities of the two cities, Mr. de Blasio has proposed raising taxes on the wealthy, whom he defines as those making more than $500,000, to pay for prekindergarten and after-school care. This may be a laudable goal, but people making $500,000 who are actually living and working in the city already pay high federal, state and local income taxes as well as property taxes. They’re not the ones buying apartments in places like One57, where two penthouses have sold for more than $90 million each. Places like One57, 15 Central Park West and Plaza Hotel are another New York entirely, one for the ultrawealthy with a primary residence elsewhere, for whom a $55 million condo is a pied-à-terre and just another place to park their wealth. Mr. de Blasio’s proposal would have little, if any, effect on them. They pay no city income tax and comparatively low property taxes even as the city’s services prop up the value of their trophy real estate.

Tax law in New York City is determined by New York State, not the city, so the mayor has only the power of persuasion. But the mayor can be a strong advocate. A tacit goal of the Bloomberg administration seems to have been to woo the world’s superrich with generous tax treatment, in what seems to be a continuing global competition with London, Hong Kong and Singapore. That’s not all that surprising, considering that Mr. Bloomberg is a billionaire himself, with a primary residence in New York City, a $20 million apartment on London’s Cadogan Square and a getaway in Bermuda. Attracting the superrich may well bolster some tax revenue and confer benefits on the city and its more ordinary residents, but the question remains, can and should they be asked to pay more?