Mr. Sanders has similarly proposed a tax on empty homes, aimed at speculators who sit on vacant properties until they become more lucrative to redevelop, or simply profitable to resell. That is a real problem in some communities, leaving neighbors to live with blight for years. But owning an empty home doesn’t necessarily make you a speculator. Someone who has put a property on the market and struggled to sell it for months might end up facing this tax.

The full proposal, emphasizing the feel-good parts of the market but not the others, is trying to have it both ways, said Jenny Schuetz, a housing economist at the Brookings Institution who has been following housing proposals from the 2020 candidates.

“In some ideal universe, people would buy homes (or rental properties) that appreciated slightly faster than inflation, allowing them to build wealth, but without housing costs rising too fast to pressure renters, deter new homeowners, or create excess capital gains,” Ms. Schuetz wrote in an email. “In that context, both rent control and flipping taxes make sense. The problem is, with any semblance of a private real estate market, land and housing values don’t behave that way.”

She is skeptical that it is even possible to design or regulate a market that gives modest returns to individual homeowners, but never gives big returns to landlords; that penalizes greedy flippers, but never harms small-time contractors; that taxes vacant homes but doesn’t punish working families.

Mr. Sanders’s ideas invite a host of questions about how they would technically work (how do we police a million landlords?) and where their legal authority would come from (would the courts uphold a national rent control law?). But these deeper questions about the kind of housing market voters might want seem worth hashing out, regardless of those other details.

Mr. Orton, the Sanders adviser, pushed back against the logic of economists that, for one, rent control discourages developers from building and landlords from renting housing that Americans badly need.

“I would say to those economists, how is that working out?” he said of the current deference to the market. “That’s what we’ve been doing. When we’ve left this to private developers, everything from the crash of the housing market to how we’ve seen gentrification just explode in some of the most vulnerable communities, to how we’ve seen people priced out of what would normally be affordable housing — this current crisis is the result of that.”