Several presidential candidates have proposed expanding federal subsidies for renters.

Senator Kamala Harris of California has proposed a new tax credit, at an estimated cost of $93 billion a year, for lower-income renters who spend more than 30 percent of their income on housing. Mr. Booker has proposed a more generous tax credit for renters that would cost about $134 billion per year, largely because it does not phase out benefits for renters whose incomes approach the maximum for eligibility. And Mr. Castro has proposed an even more expansive — and expensive — package: He would offer housing vouchers to any family making less than 50 percent of the local median income, roughly quadrupling the existing Section 8 housing voucher program, and he would offer a tax credit to any family making between 50 and 100 percent of the median income and paying more than 30 percent of its income in rent.

The four candidates — Mr. Booker, Mr. Castro, Ms. Harris and Ms. Warren — also have proposed policies to increase homeownership, particularly among minorities. Such policies, if successful, could ease the demand for rental units. But the details of their proposals vary significantly, and deserve separate consideration.

The plans for rent subsidies reflect a tendency among the crowded field of Democratic candidates to behave as if the election were an auction in which the highest bidder will claim the nomination.

The Harris plan is particularly ill conceived because she has not proposed any companion effort to increase the supply of housing. There is a surface logic to giving money to people who can’t afford to pay the rent. Increasing the demand for housing without increasing the supply, however, tends to drive up prices. A 2005 increase in the value of federal housing vouchers ended up lining the pockets of landlords, according to one study.

Mr. Booker and Mr. Castro both have proposed to increase federal subsidies for the construction of affordable housing, but the dollar figures are dwarfed by the subsidies for renters. The priorities should be reversed: Building housing should be the primary goal.

Ms. Warren has avoided any increase in rental subsidies , proposing to focus exclusively on construction. But the implicit logic, that any given dollar is best spent on building, goes too far. Increasing the supply of housing is the work of decades, and many lower-income families require rent subsidies even to afford construction-subsidized buildings.

Rent subsidies also hold promise as a tool for reducing residential segregation. Poor children raised in economically diverse neighborhoods thrive by comparison with those raised in concentrations of poverty, yet subsidized housing tends to be built in neighborhoods with high levels of poverty. Under the Obama administration, renters in some cities were offered larger vouchers if they agreed to move to areas with better schools, where housing tends to be more expensive. The early results were promising, and the program deserves to be revived and expanded. Proposals to make federal infrastructure funding contingent on land use reform also might be usefully extended by requiring affluent communities to accept affordable housing projects.