NEW YORK — For decades, the tax code has been filled with rewards for homeownership. Tax breaks encourage people to get into first homes and to trade up as they get older, building a national mind-set that you’re never quite middle class until you’ve qualified for a mortgage.

It amounts to a vast social engineering project that assumes society is better off with owners instead of renters. But the tax bill making its way toward final passage is upending that premise.

The bill will increase many homeowners’ monthly housing costs by scaling back deductions that allow them to reduce mortgage interest and property taxes. And by roughly doubling the standard deduction, it reduces the incentive to buy homes by making far fewer homeowners eligible for preferential tax treatment.

Today, a little under half of US homes are worth enough to justify itemizing mortgage interest and property taxes. Under the tax legislation, that figure would fall to close to 14 percent, according to an analysis of the plan by the online real estate marketplace Zillow.

The Republican plan, in short, is tinkering with subsidies so entrenched in the social fabric that they have become entitlements in all but name.

“It suggests a limit in the federal government’s willingness to subsidize ownership,’’ said Edward Glaeser, an economist at Harvard. “It’s also a reflection of just how expensive housing has become, and how it feels problematic to be using the tax code to support people buying houses that are this expensive or, even worse, to be encouraging housing prices to rise further.’’

President Trump on Saturday defended the Republican tax cut as a good deal for the middle class and suggested that it could lead to explosive economic growth.

The legislation, which the GOP aims to push through Congress next week, would lower taxes on the richest Americans. The benefits for most other taxpayers would be smaller. And the tax cuts are projected to add $1.46 trillion to the national debt over a decade.

But Trump said it will bring job growth. ‘‘It'll be fantastic for the middle-income people and for jobs, most of all,’’ he told reporters before traveling to Camp David for the weekend.

Both parties have long championed homeownership as a way to help people build wealth and keep neighborhoods more stable. But economists like Glaeser have been critical of the resulting subsidies.

In their view, the government has made homeownership and its financing artificially cheap through the tax code and mortgage backers like Fannie Mae. As a result, people are encouraged to take on more debt than they might otherwise — to buy bigger homes and second homes, and to plow the equity they accrue into renovations and personal spending.

This distorts the economy in a number of ways. For starters, it’s unfair: Since the benefits of these deductions get bigger with larger and more expensive homes, the bulk of the benefits accrue to wealthier homeowners in pricier markets.

This alters the landscape by encouraging more single-family homes and suburban sprawl. That prompts the government to spend more on roads and infrastructure and makes housing a bigger portion of the economy than it would be in the absence of federal help.

Construction, though, is one of the least productive industries. By funneling more of the national debt and savings into construction, the government is hindering sectors, like education and manufacturing, that have a bigger economic payoff.

All this has made homeowner subsidies, in particular the mortgage deduction, one of the rare tax breaks with critics across the political spectrum. Matthew Desmond, a Princeton sociologist who studies how eviction hurts the poor, says the deduction became the “engine of American inequality’’ because it favors higher-income homeowners.

Edward J. Pinto, codirector of the conservative American Enterprise Institute’s Center for Housing Markets and Finance, has described the interest deduction and other homeowner subsidies as a wasteful giveaway that inflates home prices and encourages people to borrow excessively.

“My basic view is if you subsidize something you’ll get more of it, and as a country we’ve been subsidizing debt,’’ he said.

The bill does retain significant subsidies, allowing homebuyers to deduct interest on mortgages as high as $750,000 — accounting for the vast majority — and up to $10,000 total in property taxes and state and local income taxes. But real estate agents have portrayed the changes as a full-blown attack on their industry.

“The final tax reform bill released punishes homeowners and weakens homeownership,’’ the California Association of Realtors said.