A Make America Great Again hat sits on the ground during a rally with President Trump in Washington, Mich., in April. (Daniel Acker/Bloomberg News)

Don­ald Trump prom­ised to save American manu­fac­tur­ing.

“It’s time to re­build Mich­i­gan, and we are not letting them take your jobs out of Mich­i­gan any long­er," Trump told thou­sands of sup­port­ers at a sub­ur­ban Detroit shop­ping mall dur­ing the 2016 pres­i­den­tial cam­paign, pledg­ing to re­ne­go­ti­ate the North American Free Trade Agreement signed into law by his op­po­nent’s hus­band. "Hil­la­ry Clinton ... will nev­er pro­tect the free­dom and jobs of the American people. … We will bring back your auto manu­fac­tur­ing busi­ness like you have nev­er ever seen it before. … My plan in­cludes a pledge to re­store manu­fac­tur­ing in America.”

Dur­ing that cam­paign, Trump also de­liv­ered a “jobs plan” speech in Monessen, Pa., a small town out­side Pittsburgh that over the past 50 years has lost its steel mills — and more than half its pop­u­la­tion. He prom­ised to re­store the faded in­dus­trial town to its smoke­stack glory. To make Monessen great a­gain.

“The leg­acy of Penn­syl­vania steelworkers lives in the bridg­es, rail­ways and sky­scrap­ers that make up our great American land­scape,” Trump said. “But our work­ers’ loy­al­ty was re­paid with be­tray­al. Our poli­ti­cians have ag­gres­sive­ly pur­sued a pol­icy of glob­al­i­za­tion — mov­ing our jobs, our wealth and our fac­to­ries to Mexi­co and over­seas. Skilled crafts­men and trades­people and fac­to­ry work­ers have seen the jobs they loved shipped thou­sands of miles away. Many Penn­syl­vania towns once thriv­ing and hum­ming are now in a state of de­spair. This wave of glob­al­i­za­tion has wiped out our mid­dle class. It doesn’t have to be this way. We can turn it all around — and we can turn it around fast.”

Mich­i­gan and Penn­syl­vania be­lieved him. Both states voted Republican for the first time since 1988. But now Trump is threat­en­ing to slap a 25 percent tar­iff on auto­mo­biles and auto parts en­ter­ing the United States, a move that the Center for Auto­mo­tive Research in Ann Arbor, Mich., says would in­crease the average car price by $4,400 and re­sult in the loss of 715,000 jobs. At a Commerce Department hear­ing, 44 out of 45 wit­ness­es, in­clud­ing Ford, General Motors and Chrysler, tes­ti­fied against the tar­iffs. Only the United Auto Workers of­fered sup­port for “tar­get­ed meas­ures to boost do­mes­tic manu­fac­tur­ing.”

Why is Trump so de­ter­mined to im­pose tar­iffs that could hurt, rath­er than save, manu­fac­tur­ing jobs? Be­cause in his nos­tal­gi­a for the heartland of yore, he mis­tak­en­ly thinks he can take the nation back to the days be­fore for­eign cars.

Trump wants to bring back the 1960s re­mem­bered at events such as the Oldsmobile Home­com­ing in my automaking hometown of Lan­sing, Mich., which gath­ers to­gether the rem­nants of a name­plate that was hard-core American iron 50 years ago (the 1966 Tor­o­na­do was Motor Trend Car of the Year), but is now as ob­so­lete as the Pack­ard. At one re­cent Home­com­ing, I met a retired autoworker who had start­ed at Oldsmobile in 1965, three months out of high school, on GM’s big­gest hir­ing day since World War II. Thanks to good un­ion wages and bene­fits, he had gotten mar­ried at age 20, built a house on three acres at age 27 and retired on a hand­some pen­sion at 55.

“We had a good wage, we had good health care, we had a good pen­sion,” he told me. “Ev­er­y­thing was there. I was there for the best years of Oldsmobile, as far as I’m con­cerned. I know many people that are pro­duc­tion, sala­ried, en­gi­neers that I graduated with, they all say the same thing. We all saw the best Oldsmobile had to of­fer. I don’t think it’ll ever go back to the way it was.”

The hey­day of American manu­fac­tur­ing, which last­ed rough­ly from the end of World War II to the Arab Oil Em­bar­go of 1973, is the fo­cus of Trump voters’ nos­tal­gi­a for American great­ness. In the mid-1960s, 90 percent of the vehi­cles sold in the United States were built by General Motors, Ford or Chrysler — double their cur­rent share. Bullitt drove a Ford Mustang and Steve McGarrett fought crime in Honolulu out of a Mer­cury Park Lane Brough­am. That was when any guy with a high school ed­u­ca­tion — the sweet spot of Trump’s dem­o­graph­ic — could walk straight from the grad­u­a­tion line to the as­sem­bly line, earn­ing en­ough mon­ey to sup­port a stay-at-home wife and a fam­i­ly. Back then, there were few im­mi­grants to con­tend with for jobs, eith­er: In 1970, the United States' for­eign-born pop­u­la­tion hit an all-time low of 4.7 percent. It was an era when Americans didn’t have to com­pete with for­eign­ers, eith­er at home or a­broad. We were win­ning. All the time. At a re­cent “Made in America” e­vent at the White House, which showcased do­mes­ti­cal­ly manu­fac­tured pro­ducts, Trump crit­i­cized pre­vi­ous ad­min­is­tra­tions for letting “our people lose their jobs to work­ers in for­eign lands” and de­clared “the era of eco­nom­ic sur­ren­der for America is over. America is fight­ing back and we’re win­ning a­gain.” When Trump prom­is­es to “bring jobs back,” his lis­ten­ers hear a prom­ise to re­store that gold­en age.

“People — es­pe­cial­ly when you go to Trump voters — they have this view of manu­fac­tur­ing that’s re­al­ly e­mo­tion­al­ly packed,” Jer­e­my Ros­ner, ex­ec­u­tive vice president at the poll­ing firm Greenberg Quin­lan Ros­ner, told NPR earli­er this year. “There’s def­i­nite­ly a huge, nos­tal­gic ’50s, ’60s, hey­day-of-America, Ro­sie-the-Riv­et­er-laden kind of thing around manu­fac­tur­ing. So people in those com­mu­ni­ties who hear Trump or who­ev­er it is talk­ing a­bout pro­tect­ing those jobs, there’s a lot of emo­tio­nal over­tones.”

Although it may have an emo­tio­nal ap­peal to his sup­port­ers, Trump’s tar­iff plan ig­nores the mod­ern glo­bal ec­on­omy, in which cars as­sem­bled in the United States use for­eign-made parts. Ac­cord­ing to the cars.com American-made index, even the most American car, the Jeep Cher­o­kee, con­tains 28 percent for­eign parts — which could be sub­ject to tar­iffs un­der Trump’s plan, and thus in­crease the cost of the ve­hi­cle. No federal pol­icy can bring back the nation’s post-World War II in­dus­trial dom­i­nance, be­cause so much of it was a func­tion of the fact that the United States was the only coun­try to e­merge from the war with any in­dus­trial ca­pac­i­ty, but Trump im­ag­ines that eco­nom­ic i­so­la­tion­ism can re-create the con­di­tions in which the United States was the world’s sole in­dus­trial su­per­pow­er.

Re­gard­less of that eco­nom­ic and historical re­ali­ty, Trump’s ap­peal to the Rust Belt’s nos­tal­gi­a for the days when a fac­to­ry job was a “pre­cious birth­right,” to quote autoworker/author Ben Ham­per’s shop mem­oir “Rivethead,” may have been de­ci­sive in his 2016 vic­to­ry. The Rust Belt is the bell­weth­er of American politics be­cause its prob­lems are too deep­ly root­ed for any pol­i­ti­cian to solve, so it keeps throw­ing out poli­ti­cians who are unable to solve them. President Barack Obama won the Rust Belt be­cause he bailed out the bank­rupt auto in­dus­try; Trump won it be­cause he prom­ised to bring back the jobs on the as­sem­bly line. At least Trump has a plan to force com­panies to move their op­er­ations back to the United States, no mat­ter how coun­ter­fac­tual it may be — and it taps into an­ger a­bout outsourcing that has trans­ferred well-pay­ing American blue-col­lar jobs to the Third World, deva­stat­ing small manu­fac­tur­ing hubs, from Flint, Mich., to Galesburg, Ill.

“My own o­pin­ion on the tar­iffs and the things he’s doing are it’s all part of a mas­ter plan to get the jobs to come home,” Frank Pitch­er, 51, a Ford work­er out­side Detroit who voted twice for Obama and then for Trump, told The Post last month. “In my 25 years, I’ve seen us start off with 145,000 work­ers and go down to 45,000.”

Don’t ex­pect Trump to dial back on the eco­nom­ic na­tion­al­ism that has proven so ap­peal­ing to manu­fac­tur­ing work­ers, even if it doesn’t end up help­ing them. Be­cause they’re con­cen­trated in the swing states of the Midwest, they’re the most im­port­ant fac­tion in his elec­tor­al coa­li­tion. These work­ers view free trade and im­mi­gra­tion as threats to their live­li­hood. Trump is their cham­pi­on. As Noah Smith wrote in Bloom­berg O­pin­ion, “Make America Great A­gain” “has come to rep­re­sent the i­de­a that a strong leader, by force of will, can re­turn the U.S. to the in­dus­trial ec­on­omy and in­ter­na­tion­al dom­i­nance of the 1950s.”

So if you end up pay­ing a lot more mon­ey for a Jeep Cher­o­kee, or a Chevy Im­pa­la, or a Ford F150, blame Trump, blame the elec­tor­al col­lege — and blame the Rust Belt’s nos­tal­gi­a for its most pros­per­ous era.