Stocks slumped on Wednesday ahead of President Trump’s announcement of tariffs on auto imports, as investors braced for levies that could disrupt carmakers’ supply chains and raise costs for consumers.

The S&P 500 fell roughly 1.1 percent for the day, and the tech-heavy Nasdaq Composite was 2 percent lower. The downturn reflected ongoing volatility in the stock market, which has been fueled by Mr. Trump’s whiplash on trade policies and concern among investors that sweeping tariffs could reignite inflation and slow down the U.S. economy.

Shares of most major automakers dropped on news from the White House that Mr. Trump would announce new auto tariffs Wednesday afternoon — and they extended their declines after he unveiled plans for 25 percent tariffs on all cars shipped to the United States. General Motors, which was down more than 3 percent when markets closed, tumbled another 5 percent in post-market trading. Toyota also fell further, while Ford’s stock, which had ended the day slightly higher, erased its modest gains in after-hours trading, plummeting about 5 percent.

Shares of Tesla were more than 5 percent lower when markets closed, but its stock inched up in post-market trading. Tesla shares gave up some gains after the Trump administration said that imported auto parts would face tariffs, too.

The stock market has dropped over the past month, a wave of selling that at one point left the S&P 500 more than 10 percent below its Feb. 19 peak. Despite some recent daily gains, Wall Street sentiment continues to be dampened by concerns that Mr. Trump’s tariffs and a trade war could push prices sharply higher, discourage consumers and damage the economy.