Former presidents George H.W. Bush and Jimmy Carter are both over 90, and still with us — making it just barely conceivable that John F. Kennedy might have lived to celebrate his 100th birthday on Monday, if he had not been assassinated in Dallas on Nov. 22, 1963.

Surely JFK would have noted a contrast between his Jan. 20, 1961, Inaugural Address and that of his succcessor Donald Trump exactly 56 years later. “Ask not what your country can do for you, ask what you can do for your country,” Kennedy instructed. His epigrammatic call for patriotic responsibility resonated in a nation of World War II and Korea veterans “tempered by war, disciplined by a hard and bitter peace.”

President Trump encouraged a sense of grievance in his very different audience, casting them and their country as victims of a corrupt “establishment” focused more on “other countries” than “the just and reasonable demands of a righteous public.” Instead of offering to “bear any burden” or “support any friend” on behalf of liberty, Trump issued a “new decree” of “America First” and claimed that “protection will lead to great prosperity and strength.”

And this was no mere tonal difference; it was a flat repudiation of JFK’s policy legacy, whether Trump intended it that way or not.

One of the things Kennedy asked of Americans was to break with their protectionist past once and for all. He spent much of 1962 campaigning for the Trade Expansion Act, a tariff-slashing measure he called “the most important international piece of legislation . . . affecting economics since the passage of the Marshall Plan.” Congress passed the bill with bipartisan support, and JFK signed it Oct. 11, four days before he learned Soviet missiles were on Cuba.

Indeed, but for the missile crisis, free trade might loom larger in JFK’s legacy. Familiar institutions such as the U.S. trade representative, “fast-track” negotiating authority and trade-adjustment assistance owe their existence to Kennedy’s law, which he presented as an act of enlightened self-interest, economic and geopolitical.

With Europe recovering from World War II and forming a single market, Kennedy feared the United States would be shut out — a rupture the Soviets could exploit — unless he and his successors could negotiate reciprocal tariff reductions, as the new law provided.

“Communist hopes for a trade war between these two great economic giants would be frustrated — and Communist efforts to split the West would be doomed to failure,” Kennedy said. “Our efforts to maintain the leadership of the free world thus rest . . . on our success in this undertaking. Economic isolation and political leadership are wholly incompatible.”

As for the impact on the United States, he argued, most companies and workers would benefit, due to increased exports and “healthy competition” from imports.

If that argument sounds familiar, it’s because a version has been offered in favor of free trade ever since, most recently in President Barack Obama’s ill-fated campaign to sell the Trans- Pacific Partnership as a job-creating counter to Chinese influence in Asia.

Trump’s election and the ascendance of Sen. Bernie Sanders (I-Vt.) in Kennedy’s Democratic Party show the waning appeal of Kennedyesque thinking.

Many voters are coming full circle to the views of the late Rep. John H. Dent, who in 1962 opposed Kennedy’s bill because he could not agree “to put any worker out of a job . . . to benefit another worker who is not an American.”

Dent, who also alluded darkly to lobbying by “registered agents of foreign countries,” was a Democrat from an industrial western Pennsylvania county, Westmoreland, that voted 55 percent for JFK in 1960 — and 63 percent for Trump in 2016.

Kennedy’s Trade Expansion Act is in the news today only because Trump is wielding an obscure provision — the exception allowing protection on national-security grounds — against metal imports from China and elsewhere.

Born in 1917, the year that America’s entrance into World War I confirmed its global rise, and a veteran of World War II, Kennedy was steeped in the lessons of U.S. experience abroad before his presidency. If anyone epitomized the foreign policy establishment that arose in the mid-20th century, and that Trump seeks to delegitimize, it was JFK.

Kennedy’s vision, though inspiring, did not adequately account for the costs, in domestic political sustainability, of any “long twilight struggle,” especially any that “asked” Americans from places like Westmoreland County to sacrifice in places such as Vietnam and Iraq. Trump, in that sense, is not only a cause of American grievance but a symptom.

Kennedy’s arguments, both for free trade and for the U.S. leadership of which it is a part, retain validity, however. The task confronting heirs to the JFK tradition is to recognize what may be real and legitimate in the sentiments Trump tapped, to adjust accordingly — and to carry on.

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