Everyone is talking about the ‘affordability crisis.’ It can’t be solved.

On Friday, he came face to face with him in the Oval Office.

While the affordability crisis might not be a Democratic “con job," as Trump once claimed, it certainly dominates the vibes, and no one embodies the vibes better than Zohran Mamdani, who was elected mayor of New York City this month.

Mamdani, like the Democratic candidates for governor in New Jersey and Virginia, campaigned single-mindedly on affordability. He was still at it Friday. Asked, with Trump at his side, about the Middle East or whether Trump is a fascist, Mamdani kept returning to the “affordability agenda" and the “cost of living crisis."

But getting elected on vibes is one thing; solving them is another. And that’s the problem with the affordability crisis. What started as a serious but short-lived spike in inflation from 2021 to 2023 has evolved into something broader and more amorphous. Like the climate crisis or the crisis of democratic legitimacy, the affordability crisis has become an umbrella term for countless loosely connected phenomena.

Like those other crises, this one defies definition and thus resolution.

Affordability isn’t a synonym for inflation, which is the rate of change in the overall price level. Inflation reached 9% in mid-2022 but was down to 3% in September. Though it is above the Federal Reserve’s preferred 2%, that 1 percentage point difference is probably not the crisis people have in mind.

Strictly speaking, affordability means having the resources to pay for goods and services at current prices. By that standard, the simplest metric is real (i.e., after-inflation) incomes. Real incomes fell behind when inflation shot up, then recovered as inflation receded and wages caught up. Real personal income was up 2.3% in the year through August, and real hourly wages climbed 0.8% in the year through September, both in line with the 19-year average.

Clearly, the affordability crisis can’t be captured by such macroeconomic measures. It is an amalgam of microeconomic irritants that vary by individual, time and place.

Because there is always something going up in price or someone whose incomes are suffering, affordability is an especially potent issue for political challengers such as Trump and Mamdani, who have a gift for zeroing in on whatever is most salient to voters.

Trump relentlessly bashed President Joe Biden and Vice President Kamala Harris, the Democratic nominee last year, over the price of groceries and gasoline. That some of it wasn’t true (“Bacon is up five times!") didn’t matter. The attacks resonated.

When Trump returned to office, the public was obsessed with eggs, whose price had doubled in the prior year as flocks were culled to contain avian flu. The price has since fallen sharply, as Trump and his surrogates regularly point out. But the public’s attention has moved elsewhere, most notably to beef and coffee.

As a campaigner, Trump loved to talk about gasoline because its price had risen under Biden (though it fell from its peaks after Russia’s full-scale invasion of Ukraine in 2022), and more production of fossil fuels was one of his central policies. Since January, gas prices have indeed dropped. But the newly salient issue is rising electricity rates.

There is nothing any elected official can do to “solve" the affordability crisis reliably. As Biden learned, the people as a whole don’t want lower inflation (i.e., prices to rise more slowly); they want prices to fall. Trump promised they would. Overall prices haven’t fallen and almost certainly won’t. For prices merely to stop rising for a year (i.e., an inflation rate of zero), would probably require a deep recession. Overall prices haven’t fallen materially since the Great Depression.

Individual actions such as reduced regulation on energy production and infrastructure and capping certain drug prices will help at the margin, but tariffs do the opposite (as Trump seems to have acknowledged by rolling some back).

Housing is especially hard to solve. It has become much more expensive since the pandemic. From 2008 through 2021, mortgage rates were abnormally low, a product of very low inflation and aggressive Federal Reserve policies, which boosted home prices. Mortgage rates have since returned to pre-2008 norms, but housing prices haven’t yet adjusted downward, so monthly payments remain high, especially for first-time buyers.

That is slowly changing. New-home prices have slipped for the past few years, existing-home prices have stopped rising, and mortgage rates are down half a percentage point in the past year.

Housing affordability is now slightly below its pre-2008 average, according to the National Association of Realtors, so room for improvement is limited. Trump wants to stack the Fed with loyalists who will slash interest rates, but that wouldn’t return mortgage costs to prepandemic lows absent a much-worse economy. And a politicized Fed would ultimately lead to higher inflation and rates.

Mamdani, like Trump, frames affordability in memorable terms. “We’re in the wealthiest city in the history of the world, and yet one in five can’t afford $2.90 for a MetroCard," he declared Friday, a convenient way to justify his proposal for free buses.

But New York isn’t expensive because of public transit. When the fare rises to $3 in January, it will have climbed an average of 1.7% annually over the past decade, below the city’s inflation rate.

In January, Mamdani memorably dove into the frigid waters off Coney Island to publicize his freeze on rents. And New York rents have gone up a lot recently, while they have stabilized or fallen in other cities thanks to a surge of supply.

New York has long been an expensive and difficult place to build housing, and while Mamdani has ideas to boost supply, some, such as a rent freeze, will likely discourage supply and vacancies.

Like Trump, Mamdani will find it easier to campaign on affordability than to solve it.

Write to Greg Ip at greg.ip@wsj.com