A fresh tariff threat from President Trump sank stocks on Thursday, pushing the S&P 500 to its fourth consecutive daily decline and reinvigorating investor worries about the outlook for the global economy.

The trading day didn’t start out so badly. Stocks rose throughout the morning, after the Federal Reserve cut interest rates on Wednesday, with the S&P 500 gaining as much as 1 percent by the early afternoon.

Then, just before 1:30 p.m., Mr. Trump said on Twitter that the United States would impose a 10 percent tariff on an additional $300 billion worth of Chinese imports starting in September.

Over the next hour, the market’s gains quickly melted away. Outside the stock market, benchmark oil prices, which are highly sensitive to the global economic outlook, plunged 7 percent and yields on government bonds tumbled.