President Trump has the right idea after last week’s elections to focus on affordability, but his first proposal to address the staggering cost of housing should be a nonstarter.

This week, Trump floated the creation of a 50-year mortgage loan — a term 20 years longer than the standard 30-year loan — to lower the cost of financing a home. Bill Pulte, Trump’s director of the Federal Housing Finance Agency, leapt to back his boss’s proposal. Pulte claimed that his agency was working to develop a 50-year mortgage for homebuyers, calling it a “complete game changer.”

Yet a 50-year mortgage would not decrease the cost of purchasing a house. Instead, by extending the term of the loan by two decades relative to a 30-year mortgage, it would nearly double the amount of interest borrowers would need to pay. The sole financial benefit to homebuyers would be in their monthly payment, which would fall slightly as the cost of their mortgage is spread across 20 additional years. In return, however, the lifetime cost to borrowers could rise by hundreds of thousands of dollars.

To illustrate, a standard 30-year mortgage of $350,000 at a 6 percent interest rate currently requires a monthly payment of about $2,100. Over the course of the loan, the borrower is expected to pay $405,000 in total interest. Now, consider the same loan but with a 50-year term. The borrower would face a slightly lower monthly payment of $1,840, or $260 less than under the 30-year loan. Meanwhile, the total cost of interest over the course of the mortgage would balloon to $755,450 — almost twice what one would pay with a standard mortgage.

Of course, most homebuyers do not remain in their properties for 30 years, let alone 50 years. But that fact makes the economics of a 50-year mortgage even worse for borrowers. Because interest payments are front-loaded, those who purchase homes with 50-year mortgages would be unlikely to gain much equity by the time they go to sell them. Take the same example of a $350,000 mortgage at a 6 percent interest rate that is held for twelve years, the median tenure of homeownership. By the time the homeowners sell their house, they will have only paid off $19,800 worth of principal beyond the initial down payment. That’s just over 5 percent of their loan’s face value, a measly amount of equity to have accumulated over twelve years.

A 50-year mortgage would also be a much riskier bet for the lender — which, in this case, would effectively be U.S. taxpayers. The body that Bill Pulte leads, the Federal Housing Finance Agency, oversees the federal government’s complex system of mortgage guarantees. The two main entities it regulates, Fannie Mae and Freddie Mac, are government-sponsored enterprises that purchase an enormous share of mortgages originated by lenders across the country.

Fannie and Freddie then assume the credit risk of these mortgages, which are ultimately backstopped by the federal government. Pulte wants to use the extensive reach of the government-sponsored enterprise to set a new market standard of offering 50-year mortgages. In doing so, however, he would need to transfer the risk of borrowers defaulting on these loans to taxpayers. The 30-year fixed-rate mortgage is already riskier than the 15-year alternative, as lenders have to assume that borrowers will maintain their ability to pay principal and interest for twice as long. A 50-year mortgage would be far riskier still, putting the taxpayers who guarantee the loans in greater financial peril.

Even setting aside the downsides for borrowers and taxpayers, a 50-year mortgage would likely worsen the very problem it claims to help. In attempting to tackle housing unaffordability, the president wants to artificially stimulate demand for homes by lowering the cost of credit without directly addressing supply. That is the same failed formula that the federal government has attempted for decades — one that has only resulted in record-high barriers to first-time homeownership.

To put a dent in home prices, policymakers must focus on expanding the supply of housing by removing burdens on construction, such as overly stringent zoning rules, building codes, and other land-use restrictions that limit what can be built and drive up costs. This is mainly a job for state and local governments, but the federal government should be aggressively encouraging a supply-side revolution in housing — perhaps by conditioning federal grants on certain reforms.

If tearing down obstacles to new home construction would be a comprehensive treatment to the nation’s housing woes, the 50-year mortgage is more of a Band-Aid. America ought to get busy building — not fiddling with the terms of its mortgage financing.