President-elect Donald Trump's threat to impose tariffs on Canada, China and Mexico on the first day of his administration could dramatically raise prices for Americans on everything from cars to clothes to oil.

President-elect Donald J. Trump said on Tuesday evening that he had selected Dr. Jay Bhattacharya, a Stanford physician and economist whose authorship of an anti-lockdown treatise during the coronavirus pandemic made him a central figure in a bitter public health debate, to be the director of the National Institutes of Health.

Leaders in Mexico and Canada responded to the president-elect’s proposed new tariffs. Observers said the plan might just be the first step in negotiations.