SKAGWAY, Alaska—Orion Hanson climbed into his Chevy Silverado pickup and donned an old Montréal Expos baseball cap.

A local elected leader in Skagway, Hanson hoped the gesture of goodwill might smooth the waters as he prepared for an important meeting with Canadian officials across the border.

“I wanted to put a Canadian flag on my truck,” he said, as he accelerated up the Klondike Highway, on a diplomatic mission to Whitehorse, the capital of Canada’s Yukon Territory.

Hanson’s challenge: to mend bonds strained by President Trump ’s 25% tariff on many Canadian imports and talk of making Canada “ the 51st state. ” In particular, Hanson wanted to preserve a $28 million binational port project the Yukon is largely funding and urge officials not to discourage their citizens from visiting Alaska.

Yukon Premier Ranj Pillai , who had a polar bear sculpture on his desk, welcomed Hanson as an old friend. But he didn’t mince words about Canadian sentiment.

“I’m going to be very blunt,” the premier said. “It’s going to be a personal decision that people are making.”

Bound together

Few states feel a rift with Canada more keenly than Alaska, which shares its sole land border with its neighbor. The capital of Juneau, like much of southeastern Alaska, is almost completely encircled by Canada. Alaska exports some $600 million a year in goods to the country.

The regions are so enmeshed that the Alaska House recently passed a bipartisan resolution opposing trade restrictions and explicitly affirming Canada’s sovereignty. “It is one divorce you can’t have,” explained Republican Majority Leader Chuck Kopp, the measure’s co-sponsor.

Up here in the North, many Alaskans feel more kinship with Canada than the “Outside,” local lingo for the Lower 48 U.S. states. Picturesque Alaskan hamlets like Haines and Skagway serve as gateways for tourists visiting the Yukon. Residents cross for cheaper goods in Whitehorse, while Canadians come south for coastal amenities. Communities unite for softball tournaments.

But Canadian visitors to Haines and Skagway have dropped —a deliberate snub.

Whitehorse resident Karen McColl, a wildlife viewing specialist, went so far as to send an email to Skagway’s mayor with the subject line, “I’m breaking up with Alaska.”

“I don’t want to punish individuals and small businesses, but right now, it’s more important for Canadians to make our voices heard,” she wrote. “Why would I want to visit a country that’s acting so aggressively towards my own?”

A popular June bike relay in Haines—typically sold out within days—now has so few Canadian bookings the town is advertising in Whitehorse. “I’ve been angry because we shouldn’t be in this place,” said Rebecca Hylton, Haines’s tourism director.

Sarah Bishop said she won’t rebuild a restaurant that burned down in Haines in December. “It was already difficult and this makes it more difficult,” Bishop said, a cool wind whipping her hair outside the tiny Haines airport. Inside the lobby, a video board gushed, against a pink backdrop: “Haines Loves Canada.”

In Skagway, a stunning town of 1,100 nestled beneath a glacier, the recent binational Buckwheat Ski Classic saw few Canadians sticking around afterward to party or shop. “My biggest fear is losing relationships,” said Mike Healy, owner of Skagway Brewing Co. Canadians typically comprise a fifth of his sales this time of year. In March, these numbers fell 75%.

Hana Schindler, who lives on a hillside above Skagway with her husband, Bruce, said she tried to apologize to a close friend in Whitehorse on behalf of the U.S. “She said, ‘We understand that but we’re not going to Skagway,’ ” Schindler recalled.

Bruce Schindler—who collects mammoth tusks to carve—fears his main business of buying gold in the Yukon will be destroyed by high tariffs.

A small-town diplomat’s journey

Hanson, a builder and Skagway assemblyman, crosses the border regularly for supplies in Whitehorse. As he pulled up to the Canadian border entry on a recent afternoon, there appeared to be a palpable chill as a customs agent admonished him for approaching without permission. “If I need to take my firearm out, you’re not in the best position,” the agent warned.

Hanson shrugged it off. But he said he hadn’t encountered that kind of aggression before.

As he continued into a wilderness of snow-capped mountains and boreal forest, Hanson talked about one of the Yukon’s more famous residents. Trump’s grandfather, a German immigrant named Friedrich Trump, joined a stampede of prospectors in the 1898 Klondike Gold Rush, operating a restaurant and brothel in the Yukon.

“The Trump empire started right here,” Hanson said.

In Whitehorse, after picking up dog medicine for a friend, Hanson pulled into a slushy parking spot, for his meeting with Yukon officials about the binational port project.

Inside, Hanson met with the premier and Justin Ferbey, a Yukon deputy minister who quickly thanked him for testifying in Juneau in support of the House’s pro-Canada resolution.

Hanson looked at the minister and asked if it was true, as he had heard, that the Yukon Territory was considering a punitive toll for Americans on the Klondike Highway.

Ferbey smiled and said no. “We’re still bullish and very welcoming,” he assured Hanson. More good news followed: Funding for Skagway’s port upgrade—to handle shipments of ore from mines in Canada—wouldn’t have to go through Ottawa and potentially get tangled in federal politics. The money would instead go through Yukon’s legislature under a proposed agreement between Skagway and the territory.

“What we’re looking at right now is an agreement between Skagway and the territorial government,” Ferbey said.

The meeting ended with laughter. Hanson went to complete his errands, which included stopping at KFC for two buckets of chicken (cheaper in Canada) and at a hardware store. Canadian customer Maurice Ouimet was walking out with new wooden dowels for an art project.

“We don’t want these problems for either side,” he said. “We’re having a little spat.”

Write to Jim Carlton at Jim.Carlton@wsj.com