Stocks shook off morning losses on Monday and surged in the afternoon to send the Standard & Poor’s 500 index to its best day in a week. It was the latest turn for a market suddenly prone to quick shifts — day to day as well as hour to hour — as investors question whether President Trump will really risk a trade war.
The S&P 500 lost as much as 0.6 percent shortly after trading began, only to finish the day 1.1 percent higher after rising 29.69 points to 2,720.94. It’s reminiscent of what happened Friday, when stocks reversed course on speculation that Trump was only making an opening bid when he promised to impose stiff tariffs on imported steel and aluminum.
The Dow Jones industrial average jumped 1.4 percent, to 24,874.76, and the Nasdaq Composite gained 1 percent, to 7,330.70.
Trump took to Twitter again on Monday to defend the tariffs, which have riled trading partners around the world and already sparked talk of retaliation to heighten the worries about a possible trade war. He highlighted trade deficits with Canada and Mexico and said tariffs ‘‘will only come off if’’ a new free-trade agreement between the three countries is signed.
Later in the day, House Speaker Paul Ryan said that he was ‘‘extremely worried’’ about the consequences of a global trade war and urged the White House ‘‘to not advance with this plan.’’
‘‘It’s incredibly difficult to try to understand the whims of this current administration and to try to make forecasts,’’ said Emily Roland, head of capital markets research for John Hancock Investments. If a trade war does occur, it would hurt the global economy and the healthy profit growth that companies have been producing, two of the big drivers for the market.
‘‘But right now, we think the impact should continue to be modest, as long as it’s all talk and no action,’’ she said.
Boeing offered a good example of how quickly the market shifted. The aerospace giant got the majority of its revenue from outside the United States last year, so it would be hurt if countries put up more barriers to global trade. Boeing was down as much as 2.3 percent in the morning before ending the day up 2.3 percent.
From its low point of the day to its high, the S&P 500 index carried investors through a swing of 1.9 percentage points. It was the fifth straight day with a gap of more than 1.5 percentage points, as trading has become much more wild since the market’s placid, record-setting run from 2017 into January. During that period, the typical day saw the S&P 500 drift just 0.5 percentage points from its low point to high.
The biggest gain in the S&P 500 came from XL Group, which surged after AXA said that it will acquire the insurance and reinsurance company for $15.3 billion. XL Group stock surged 29.1 percent.
In overseas markets, Europe was mostly higher, with Italy an exception after elections there saw no single party emerge with a majority in Parliament. That raises uncertainty about how closely Italy will work with the rest of the European Union.
France’s CAC 40 rose 0.6 percent, Germany’s DAX gained 1.5 percent, and the FTSE 100 was up 0.7 percent in London.
Japan’s Nikkei 225 fell 0.7 percent, South Korea’s Kospi dropped 1.1 percent, and the Hang Seng in Hong Kong lost 2.3 percent.
The yield on the 10-year Treasury rose to 2.88 percent on Monday from 2.87 percent late Friday.