It’s one of the oldest platitudes in American politics — and one of the few ideas that both Democrats and Republicans these days seem capable of agreeing on: the existential importance of buying American.

“When we purchase products made in the USA, the profits stay here, the revenue stays here, and the jobs — maybe most importantly of all — they stay right here,” President Trump declared in 2017.

Sen. Bernie Sanders has made “expanding ‘Buy American’ policies . . . that will increase jobs in the US” a key component of his 2020 platform.

New York’s Gov. Cuomo got into the act at his Jan. 8 State of the State address. “Let’s invest directly in our greatest asset, our workforce, by making New York’s Buy American law permanent once and for all,” he said.

The idea has been enshrined in federal law since 1933, when GOP President Herbert Hoover signed the Buy American Act during his last full day in office. It was one Republican policy that Franklin Delano Roosevelt, his Democrat successor, never sought to undo.

There’s just one problem: In today’s interconnected world, it’s all but an empty promise, Fred Hochberg contends.

In “Trade Is Not a Four-Letter Word” (Avid Reader Press), Hochberg shows how some of America’s most iconic products and industries could not exist without the global market.

A central example is the American auto industry. “The process of building cars has become so globalized that realistically, no country can produce a quality, affordable car entirely on their own,” writes Hochberg — who headed the US Export-Import Bank during the Obama administration.

In fact, he says, “There’s no such thing as an American car.”

The National Highway Transportation Safety Administration (NHTSA) tallies made-in-America ratings for every make and model of automobile sold in the United States, based on the amount of domestic parts, labor and assembly in each vehicle.

‘Without the ability to bring in foreign engines, transmissions, [and] shock absorbers . . . it only took three days before there were no American manufacturers capable of assembling a single car’

By that measure, the most American car on the road in 2018 — with 75 percent American content — was Japanese: the Honda Odyssey, the popular minivan assembled at a 4,500-employee plant in Lincoln, Alabama.

That same year, the least American car was a Chevrolet — the Spark, a subcompact assembled in South Korea with a Japanese transmission. A measly 1 percent of its components originated in the United States.

In three of the last five years, in fact, the most-American car sold in America has carried a foreign nameplate, with Toyotas, Kias and Acuras all filling out the Top 10 of the NHTSA’s list. In 2018, a US-based car didn’t make the cut until number 13 — the Chevrolet Corvette, with 67 percent American content. (To be fair, preliminary 2019 numbers have a Dodge model, the 76-percent American Grand Caravan, on top of the heap.)

It’s a drastic change from the norm of just a decade ago. In 2010, 11 models sold in the US — including the Ford Explorer, the Mercury Grand Marquis, and the Mazda Tribute — were 90 percent or more American.

As recently as 2012, it was possible for US consumers to buy a 95-percent American vehicle. Sure, that car was the Toyota Matrix, assembled in Canada with a transmission made in Japan, but still.

No car since has come anywhere near that pinnacle of all-American content. Most contain far less: Our quintessential national vehicle, the Ford F150 pickup — far and away the country’s top seller, with 897,000 sold in 2019 — is a pitiful 56 percent American under the hood.

But that’s nothing to be ashamed of, Hochberg insists.

“Foreign parts ultimately make our cars better, cheaper and more reliable,” he argues. Decades’ worth of foreign competition — starting with Germany’s Volkswagen, which built a Mexican assembly plant in 1961 to construct its iconic Beetles and buses — have spurred US car companies to up their game.

“By forcing us to innovate and grow stronger,” new production techniques and engineering methods pioneered by foreign automakers “may have saved the American auto industry and the jobs it supports,” Hochberg says. “Cars . . . are a perfect demonstration of how global trade has led to better products.”

Multinational pacts like the North American Free Trade Agreement (NAFTA) and many others have encouraged companies to develop global supply chains — that is, webs of raw materials, natural resources, parts, inventions and labor that fuel the creation of a finished product.

The supply chain for Apple’s iPhone, for example, brings together materials and components from six different continents. Touchscreen glass from Corning, NY; internal gyroscopes designed in Geneva, Switzerland; the rare-earth mineral tantalum, mined in Rwanda and the Congo, needed to power tiny circuit boards; and more are all shipped to China for assembly.

Automotive supply chains are only slightly less intricate, but are just as reliant on overseas components. As proof, Hochberg points to one of the nasty side effects of the 9/11 terror attacks, when the government temporarily sealed the US border.

“Without the ability to bring in foreign engines, transmissions, [and] shock absorbers . . . it only took three days before there were no American manufacturers capable of assembling a single car,” he writes.

Some critical resources simply don’t exist within US borders — rubber for tires and rare-earth minerals for electronics, to name two.

Other vehicle components can be produced more cheaply and efficiently by overseas companies that specialize in specific parts, Hochberg says. The company Rassini, based in Puebla, Mexico, is the top supplier of leaf springs that allow American pickup trucks to handle their heavy loads. The company’s singular focus — and its lower labor costs — make their parts safe, high-quality and affordable.

To Hochberg, that’s a capitalist success story. To Trump, Sanders, and their populist supporters on both the left and the right, it’s the kind of job-destroying outsourcing that has hollowed out America’s manufacturing base.

In the 2016 presidential election, worries about outsourcing — “part of a global race to the bottom to boost the profits of large corporations and Wall Street,” as Sanders put it — drove fiery opposition to the Trans-Pacific Partnership (TPP), a huge 11-nation trade pact that President Barack Obama’s administration had negotiated. Trump pulled the plug on TPP within days of his 2017 inauguration.

Hochberg laments that decision, seeing it as a turn toward protectionism that will only hurt the US economy in the long run.

In the auto industry, as in countless others, trade — and the job creation that follows — goes both ways, he writes. For example, German-owned BMW is now the largest US auto exporter by value. Its South Carolina plant turns out precision-engineered, American-made SUVs for the German-owned company’s European market.

And in Kentucky, a plant built by Toyota to manufacture the Camry, America’s best-selling passenger car of this century, is said to have created more assembly-line jobs than any other model.

“All told, supposedly ‘foreign’ automakers employ about 130,000 Americans,” Hochberg says.

“Trade is rarely a zero-sum game,” he argues. “When we buy from other countries, and their economies improve, they in turn are able to buy more from us — supporting more jobs and industries here at home.”