With his big win in Florida Tuesday night and the end of Marco Rubio's run, Donald Trump is closer than ever to becoming the Republican presidential nominee. What should the world do about that?
The obvious answer is: nothing. Foreign leaders do not, typically, inject themselves into the election campaigns of other nations, especially friendly ones, for the simple reason that things could go badly if the wrong guy won.
"You risk having four years or eight years of frigid relations with a country that you need to deal with a lot," observes Paul Heinbecker, a distinguished fellow at the Centre for International Governance Innovation and former Canadian ambassador to the United Nations.
"If the Canadian government were to be on the record as interfering in an American election," he said Tuesday, "we could expect that the White House would do us no favours for however long that person was there."
Besides, Mr. Heinbecker observes, voters would be so angry at the interference that you could drive some of them to the side you oppose. Which may be why Justin Trudeau refused to criticize Mr. Trump when he was in Washington last week.
But Donald Trump is no ordinary candidate. A President Trump would, but for the will of Congress, expel millions of illegal Latino immigrants, prohibit Muslims from entering the United States, terminate the North American free-trade agreement (NAFTA), impose a tariff on Chinese imports and dispense with signed or planned trade agreements with Pacific and European nations. And he thinks climate change is a hoax.
The question then is whether there is a moral case, along with a case of self-interest, for other nations to warn the American electorate about how damaging a Trump presidency could be for the United States in the world.
Jacob Parakilas, assistant head of the U.S. and the Americas Program at Chatham House, a London-based think tank, believes intervention can be justified and useful, if done properly.
"The appropriate response for leaders outside the U.S. is not to weigh in on the totality of someone's campaign, but as and when a particular issue is raised, to be able to offer perspective from abroad," Mr. Parakilas said in an interview.
It would be completely inappropriate for a European leader to urge Americans not to vote for Mr. Trump. But it might be appropriate for that leader to point out the implications of a specific proposal on economic relations between the United States and Europe, or on NATO solidarity.
Australian Prime Minister Malcolm Turnbull might talk about how scrapping the Trans-Pacific Partnership would affect relations between the United States and the 11 other nations that laboured together to forge it.
Indonesian President Joko Widodo would be entirely within his rights to warn that a proscription against Muslim Indonesians entering the United States would damage relations between the two countries.
Any world leader could legitimately worry in public that a U.S.-led tariff war would imperil the global economy, and that the United States completely abandoning the fight against climate change would imperil the planet.
And Justin Trudeau would have every right, and perhaps even a duty, to observe that abandoning NAFTA would harm both American and Canadian business. He might even question how a ban on Muslim immigration would affect Canada-U.S. efforts to harmonize continental security.
"When a presidential candidate, a serious presidential candidate no less, in the U.S. makes a statement that suggests something that would upend relations with an allied country, in particular, it is entirely appropriate for the official representatives of that country to offer their assessment of what that would mean for the alliance," Mr. Parakilas observes.
But "it's a fine line," warns Mr. Heinbecker. Trade and security issues are at the centre of the U.S. election campaign. Any intervention by Mr. Trudeau, however measured, might do more harm than good. Mr. Heinbecker would advise the Prime Minister to simply declare his faith in the wisdom of the American electors and move on.
So Mr. Trudeau faces a difficult choice: to give voice to the indignation that millions of Canadians feel toward Mr. Trump's divisive politics and rhetoric; to make specific observations about specific policy proposals; or to let silence be the better part of candour.