When asked what their city would be without auto-parts makers, Pauline Ridley and Colleen Barrette, two union officials, quickly replied, “A ghost town.”

President Trump’s tariff war against Canada has unleashed widespread anxiety in Windsor, Ontario, the country’s auto-making capital. Much of it has focused on the fate of large vehicle assembly plants.

But the concern is just as high, or higher, throughout the roughly 100 smaller auto-parts plants in Windsor and the surrounding county that employ some 9,000 workers. By comparison, about 5,400 people work for the three auto factories in Windsor.

Many parts makers are small businesses without the financial cushion that auto giants can lean on to soften the blow from the 25 percent tariffs Mr. Trump applied to imported autos and some auto parts.

At KB components, where Ms. Ridley is the union chairwoman, about 100 people are currently laid off, leaving nearly 400 workers at the company’s three plants in Windsor molding plastic parts for Toyota, Ford and the electric vehicle makers Rivian and Lucid.