In a speech this month, Prime Minister Justin Trudeau of Canada, clearly annoyed by import tariffs levied by President Trump and his threats of making Canada “the 51st state,” suggested that Canadians might act individually to respond to the affronts.

“Now is also the time to choose Canada,” Mr. Trudeau said, adding, “It might mean changing your summer vacation plans to stay here in Canada and explore the many national and provincial parks, historical sites and tourist destinations our great country has to offer.”

In the weeks since, it appears that at least some Canadian citizens are taking his directive seriously.

“I’ve decided that I will no longer be traveling to the U.S. unless it’s absolutely necessary to go,” said Harold White, 72, who lives in Quebec.

Mr. White, a lawyer, said he had canceled an annual summer trip to Maine, one that he has made every year for 60 years. Over the decades, he made friends with local residents there, he said, whom he does not expect to see for the next four years. Instead, he and his wife plan to travel to Spain and in later years, make road trips across Canada.

“It pains me to think that I’m not going to Maine or to Cape Cod or even to New York City for a vacation in the near term,” Mr. White said, noting that he continued to travel to the United States during President Trump’s first term. “But this time around, really, I feel like Canadians have been slapped across the face by Trump.”

According to the U.S. Travel Association, a nonprofit group that represents the U.S. travel industry, Canadians made 20.4 million visits to the United States last year and were responsible for $20.5 billion in spending. A 10 percent decline in Canadian visitors would amount to a $2.1 billion loss, the group said.

“We have seen that people are starting to pivot away and avoid the U.S.,” said Alexis von Hoensbroech, the chief executive of WestJet, the second-largest airline in Canada. “We see also an increase of bookings into Mexico, into the Caribbean, into other non-U.S. destinations.”

Mark Galardo, a vice president for Air Canada, the country’s largest airline, said that it would be adjusting its schedule from March onward in order to “derisk” the situation.

“We are anticipating proactively that there could be a slowdown,” he said in a statement.

Florida, California, Nevada, New York and Texas are the most visited U.S. states by Canadians. Those states could experience declines in revenue in their retail and hospitality sectors from a Canadian travel boycott, according to U.S. Travel.

However, in a statement, Geoff Freeman, the group’s chief executive, said that the United States received the largest number of international visitors ever in 2018, in the middle of the first Trump Administration, and that what travelers say and what they do are often different.

“If we have a decline in travel from any specific destination, we will share that information where it needs to be shared and work with the administration to solve for that problem,” Mr. Freeman said. “We do want them coming to the United States. If that’s not happening, then we’ve got work to do.”

In a statement, a spokeswoman for Flight Center Travel Group, an international travel agency, said that the company has seen a “softening in cross-border travel bookings” from Canada to the United States — a shift that has been gaining momentum since November. The Canadian dollar is weak compared with the U.S. dollar, the statement said, and that existing reluctance was exacerbated by the announcement on import tariffs. (On Thursday, President Trump insisted that the tariffs would go into effect next week.)

Amra Durakovic, the spokeswoman, said that Canadians’ “desire for travel” remained strong, with many prioritizing other destinations over the United States so that travelers could “make the most of their journeys.”

“Looking ahead, we remain hopeful that travel and trade between Canada and the U.S. will soon resume with the confidence and ease both countries have long enjoyed,” Ms. Durakovic said in the statement.

In the United States, state tourism boards are bracing for potential effects.

Sara Otte Coleman, the tourism and marketing director for North Dakota’s department of commerce, said that the state, which borders Canada, had paused its paid marketing in Canada until officials could “better understand the sentiment about traveling to North Dakota.”

Tim Chapman is the chief executive of the International Peace Garden, a park that extends across the border between the Canadian province of Manitoba and North Dakota.

He said he has been receiving emails from Canadian visitors who said they no longer planned to visit the tourist attraction, which usually draws around 150,000 visitors a year.

In one exchange, Mr. Chapman said, a Canadian told him she could not visit because of the rhetoric out of Washington. He said that he explained that the garden was a nonprofit that needed support from visitors and that he felt she understood.

“The Peace Garden has always stood for and advocated for the peace and cooperation of our two countries,” Mr. Chapman said.

“Even though we don’t have a lot of control about what’s being said, we can still be a place where people can come together, because the vast majority of Americans and Canadians do really value that friendship and longstanding cooperation.”

Mr. von Hoensbroech, the WestJet executive, said that he did expect cross-border travel to pick back up at some point. He noted that this was common in the travel industry — short-term reactions to current events that ultimately return to general stability. Still, he said, the response was unique.

“I haven’t seen anything like this,” he said.

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