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 United States, 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 traveling to the United States are preposterous; in practice, they would be catastrophic.

On foreign policy, 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 programs 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.