Mr Donald Trump can be seen as a talented demagogue, or as the manifestation of deep pathologies in the body politic, but he is also the bearer of ideas - crudely framed and sometimes incoherent, but ideas nonetheless. Nowhere is this more true than on foreign policy.
In the address he delivered on this subject last month, some elements could have been uttered by any mainstream American politician in the last half-century: that the United States should have the world's most powerful military; that we desire to live peacefully with all nations, including Russia and China; that Iran cannot be allowed to develop nuclear weapons; that Israel is a close friend; that restraint is a hallmark of strength.
Other themes echoed views expressed by President Barack Obama and his advisers: that our allies are free riders; that the Middle East is a mess to be shunned; that it is time for "nation building" at home; that the foreign policy establishment is filled with over-educated incompetents rightly despised by a superior president and his aides.
And then there was the distinctively Trumpian touch: his slogan "America first" invoking the notorious movement before World War II that included not only traditional isolationists but also Nazi sympathisers.
Fundamentally, much of the difference between Mr Trump and Mr Obama reflects style rather than substance. Mr Obama came in scorning what he saw as the misguided "freedom agenda" of the Bush administration and determined to cut deals with President Vladimir Putin of Russia and, as we now know, with Iran under Ayatollah Ali Khamenei.
Mr Trump merely takes these views some steps further and decibels louder. He rejects what he terms "ideology" in foreign policy, including longstanding American commitments to democratically elected governments and civil liberties. He admires Mr Putin, thinks free trade is a synonym for bad deals and scoffs at a unique American role as guarantor of world order.
Voters should examine Mr Trump's statements closely not just because of what they mean for the Republican Party, but what they imply for the two-generation-old American foreign policy consensus. Even in this era of partisanship, there has been a large measure of agreement between the two parties, cemented by officials, experts and academics who shared a common outlook.
That outlook held that American interests were ineluctably intertwined with American values, and that when possible, each should reinforce the other, as when the promotion of liberty and human rights helped to weaken the Soviet Union. An open trading order and an unflinching commitment to alliances, as indispensable as they are sometimes frustrating, have also been axiomatic.
Republican foreign policy veterans like me who have vehemently opposed a Trump candidacy have done so on multiple grounds, beginning with his disdain for the norms of the Constitution. But we also believe that Trumpism in foreign policy is dangerous because of its belligerent nationalism, self-absorption, disdain for allies and comfort with the authoritarian leaders of the day.
Mr Trump's temperament, his proclivity for insult and deceit and his advocacy of unpredictability would make him a presidential disaster - especially in the conduct of foreign policy, where clarity and consistency matter. His claim that he would compel Mexico to pay for a wall separating it from the US, his desire to turn alliances with Europe and Japan into giant protection rackets, his proposals to discard both law and basic decency through extensive use of torture and by barring Muslims from travelling to the US are preposterous; in practice, they would be catastrophic.
On foreign policy, Mrs Hillary Clinton is far better: She believes in the old consensus and will take tough lines on China and, increasingly, Russia. She does not hesitate to make the case for human rights as a key part of our foreign policy. True, under pressure from her own left wing, she has backtracked on the Trans-Pacific Partnership, a set of trade deals that supports American interests by creating a counterbalance to China, and American values by protecting workers' rights. But she might edge back toward supporting it, once in.
So why not vote for her? If the choice were simply between her and Mr Trump, I would - as would some other Republican foreign policy veterans. But most of us hope for a third candidate whose character we can trust, with domestic programmes we can support. Others will simply refuse to vote; and some, no doubt, will reluctantly sign up with Mr Trump, hoping to mitigate what they cannot change.
But foreign policy experts influence no voting bloc and carry no weight in a general election. Our real task is longer term, and indeed bipartisan.
This campaign shows that the foreign policy consensus that has framed this country's work overseas since 1950 is in peril. The left wing of the Democratic Party believes in it no more than does Mr Trump. That consensus, with its attempt to reconcile values and interests, prudence with action, needs to be articulated and championed. The public must hear why American leadership abroad is essential to our prosperity and freedom at home.
There is a wide gulf between those who have thought hard about and worked on the challenge of American global leadership and those who assure the American people that foreign policy can be reduced to "don't do stupid stuff". Today, the Trump and Obama versions of that sentiment are ascendant. It is the task of those of us in the foreign policy field, Republican and Democrat alike, to make the case that they are profoundly, dangerously wrong.
NEW YORK TIMES
•The writer, author of the forthcoming The Big Stick: The Limits Of Soft Power And The Necessity Of Military Force, was the counsellor of the US State Department from 2007 to 2009.