“Everyone is running their business thinking: ‘This is life as we know it’,” said an adviser to chief executives of several large U.S. companies with significant China operations. Executives are unlikely to view the president’s tweets as an edict, said the adviser who declined to be identified. Rather, they are more likely to see it as yet another salvo in the continuing tussle over trade policy and “evidence things are getting hotter now.”

“This is bonkers,” Aaron Levie, chief executive of cloud computing company Box Inc., tweeted in response to President Trump’s call for companies to look for alternatives to China. Mr. Levie has previously spoken out against Trump policies, although Box doesn’t have significant exposure to China…

“There is no other easily identifiable supplier or manufacturer for a lot of parts,” said Eric Klein, a partner at venture-capital firm Lemnos, which invests in hardware startups. Shifting production out of China would be a complex process that would entail re-engineering production facilities, requiring a company to halt or slow production for months, he said.

“It would be financially devastating to a startup that isn’t sitting on a pile of capital,” said Mr. Klein. “The smaller the company, the more fragile their economic state is and changes like this are massively disruptive.”