Since 1988, the hulking presses at Lanex Manufacturing on the edge of Windsor, Ontario, have been stamping out door strikers, folding-seat latches, tailpipe hangers, frame braces and other prosaic bits of metal that make their way into vehicles ranging from Corvettes to Honda minivans.

But, these days, worries about the future permeate the plant as President-elect Donald J. Trump prepares to enter the White House. He has threatened to impose a 25 percent tariff on all goods exported from Canada to the United States. In Windsor, that would ravage its lifeblood: automobiles and everything that goes into them.

“Everybody’s waiting for the next shoe to drop,” Bruce Lane, the president of Lanex, said in its boardroom, whose walls were made of painted concrete blocks. “If Windsor lost its automotive business, Windsor would not survive.”

Few Canadian cities are as acutely aware as Windsor of the integration of the two countries’ economies. The city sits just across the Detroit River from Detroit, and Canada’s maple-leaf flag often flies next to the stars and stripes there. And no industry has been interwoven across the border for as long as auto making.

“These workers here in Windsor are more exposed to trade with the United States than anyone else,” Prime Minister Justin Trudeau said at a steel plant during a recent visit to the city.

Mr. Trump, he added, “is proposing tariffs that would damage not just people here in Windsor but people right across the country and indeed in the United States.”

Windsor’s two major landmarks are shared with Detroit: the $5.7 billion Gordie Howe International Bridge, scheduled to open this year, and the 96-year-old Ambassador Bridge, which carries about $300 million in cross-border trade each day. Of Canada’s $440 billion in annual exports to the United States, only oil and gas generate a larger amount than cars, trucks and auto parts.

But with Canadian officials taking Mr. Trump at his word that he will follow through on his threat of tariffs, Mr. Lane and others in the auto industry are already bracing for the potential fallout.

George Papp is the chief executive of Papp Plastics, whose headquarters sits near the imposing new suspension bridge. He said his U.S. customers, mainly automakers, would simply invoke the terms of contracts he has with them and deduct the cost of tariffs from the amount they pay him.

“Who’s going to take the hit?” Mr. Papp said. “Me, and people like me and companies like mine.”

Flavio Volpe, the president of the Automotive Parts Manufacturer’s Association, a Canadian trade group, estimated that most of his members had single-digit profit margins and that the tariffs Mr. Trump was threatening would be ruinous.

The intertwining of the auto industry across the two countries was cemented in 1965 when Canada and the United States reached an agreement that effectively eliminated the border for the industry. Today, 90 percent of cars and trucks made in Canada are sent to the United States, primarily by train.

At Lanex, small metal parts that few motorists will ever see are forged into shape by upward of 600 tons of pressure by the firm’s presses. Their journeys illustrate how enmeshed the two countries’ auto industries have become.

As a small supplier, Mr. Lane does not deal directly with carmakers, but sells his goods through larger parts makers. Seat-locking hooks that Lanex makes for Honda minivans are sent to a plant elsewhere in Ontario, where they are fitted with other parts and then shipped to an assembly line in Alabama that belongs to Honda, a Japanese company.

Mr. Lane’s factory has sent parts to Michigan for heat treating, brought them back to Windsor for more machining and then sold them to a U.S. company.

“Windsor is used to going back and forth across the border,” Mr. Lane said. “It’s like just like getting up out of bed in the morning.”

The turmoil from possible tariffs comes at an already difficult time for Canada’s auto business. Many auto-parts manufacturers have yet to see their business return to levels from before the coronavirus pandemic because of lagging car sales. In 2020, Lanex had about 60 employees working on two shifts, but it now has about two dozen employees running a single shift.

The anxiety is particularly acute in Windsor, which has a metropolitan population of roughly 484,000. Aside from cargo trucks rumbling across the Ambassador Bridge, the city’s most obvious automotive symbol is a giant Stellantis factory that produces Chrysler Pacifica minivans as well as Dodge Charger muscle cars.

A city within the city, the European-based Stellantis employs 4,500 workers at the factory. Aided by billions of dollars in Canadian subsidies, it is building a battery plant in a joint venture with the South Korean company LG in Windsor and recently spent 1.89 billion Canadian dollars (about $1.3 billion) to retool its assembly plant to make electric vehicles alongside gasoline-powered ones.

But, like many auto makers, Stellantis is now in a slump as it struggles with the transition to electric vehicles and with competition from China.

James Stewart, the president of the local union that represents Windsor’s Stellantis workers, said he did not believe a large tariff would necessarily deal a fatal blow to Stellantis’s operations in Windsor, given how much the company had invested.

But with so much of Windsor’s economic well-being intimately tied to trade with the United States, Mr. Stewart said, tariffs would deal a heavy blow, including the closing of businesses, layoffs and production cuts.

“We’re a suburb of Detroit; we’ve always felt that way,” he said, adding that Windsor seemed to be “under attack and for no reason.”

Mr. Trump initially characterized tariffs as a way to prod Canada and Mexico into better securing their borders to tamp down the flow of undocumented migrants.

But he also mused about making Canada the 51st state, noting that the United States was heavily invested in Canada’s military defense, and threatened to use economic force annex it. He has also vented about what he describes as the “subsidizing’’ of Canada by the United States, an apparent reference to the U.S. trade deficit with Canada, largely because of oil and gas imports.

The Trudeau government is expected to detail how it would retaliate against any U.S. tariffs on Monday, the day Mr. Trump is to take office.

But Canada’s comparatively small economy makes it difficult for the country to inflict substantial economic harm on the United States, though levies against specific products could hurt individual states. Retaliatory tariffs would also drive up prices in Canada.

Back at the Lanex plant, Mr. Lane said that, by pure coincidence, the company had been embarking on a “secret” manufacturing project unrelated to automobiles and that had unexpectedly become a potential hedge against tariffs. He declined to offer any details to avoid tipping off competitors.

Mr. Papp, the plastics-company owner, said that even though he would oppose tariffs, which would hurt his business, he was a fan of Mr. Trump and understood why the president-elect had argued that tariffs were needed to help rebuild industry in the United States.

Regardless of what happens, Mr. Papp said, Canada and the United States will always remain unshakable allies.

“You can’t separate our countries,” he said. “They’re bolted together.”