Stocks tumble in worst day since May, as tech shares slide and bond yields climb.

The prospect of the Federal Reserve not reaching as deep into its bottomless pockets is starting to hit home for investors.

The S&P 500 tumbled 2 percent on Tuesday — the worst one-day slide for the benchmark U.S. index since May — as investors faced the expected wind-down of the enormous bond purchases the central bank has made since the start of the pandemic.

“The deep sell-off highlights the extent of the nerves in the markets surrounding the moves of the Fed,” said Fiona Cincotta, senior financial markets analyst at Forex.com.

The coming slowdown of bond purchases is a sign of the Fed’s confidence that the economy is recovering from the upheaval of the pandemic. But, Ms. Cincotta noted, other factors are still making Wall Street wary.

“There’s also a combination of rising energy prices, concerns that inflation could be more entrenched in these elevated levels and the fact that consumer confidence is slowing,” she said.

The tumble extended into the Asian trading day on Wednesday, though investors signaled that confidence might be returning.

Stocks in Japan were down more than 2.6 percent midday. But losses in other Asian markets, like Hong Kong and mainland China, were more moderate. Futures markets were signaling that Wall Street would open modestly higher.

The trigger for Tuesday’s tumble, which cut across sectors, was a rise in the yield on the benchmark 10-year Treasury note. With the Fed preparing to slow its purchases as soon as November, investors have been selling off bonds before demand ebbs. On Tuesday, that pushed the 10-year’s yield up to 1.54 percent, its highest level since June.

Even though the Fed has said it doesn’t plan to increase interest rates for months or years, government bond yields are the basis for borrowing costs across the economy. When bond prices fall, yields rise — a move that can hinder the stock market’s performance because it makes owning bonds more attractive and can discourage riskier investments.

Tech stocks, which are particularly sensitive to the prospect of higher interest rates, were hit hard on Tuesday. The tech-heavy Nasdaq composite fell 2.8 percent, its biggest drop since February.

Higher rates would make borrowing more expensive for smaller companies, and the jump in yields was a blow to shares of several high-flying stocks. Etsy, the online craft marketplace, dropped 6 percent, and Shopify fell more than 5 percent. Both companies have soared during the pandemic.

“With tech stocks, you’re betting for a company to have a breakthrough years from now,” said Beth Ann Bovino, the chief U.S. economist at S&P Global. “If interest rates go up today, that value that you receive years from now is discounted.”

The biggest technology stocks — particularly Amazon, Apple, Microsoft, Google and Facebook — have a vast pull on the broader market and helped drag down the S&P 500. Apple fell 2.4 percent and was the best performer of the tech giants. Amazon dropped 2.6 percent while Microsoft, Facebook and Google were down by more than 3.5 percent.

But the declines cut across many sectors. Energy stocks were the exception, rallying after oil prices climbed early in the day. Schlumberger, ConocoPhillips, Halliburton and Exxon Mobil were among the best-performing shares in the S&P 500, though some of their gains faded as oil futures turned lower in the afternoon.

The Delta variant of the virus remains a concern for investors, while persistent supply-chain bottlenecks have affected everything from auto production to school lunches. In Washington, lawmakers remain deeply divided over spending on infrastructure and expanding social programs.

And another pressing fight is brewing over raising the nation’s debt limit — a dispute that could trigger a government shutdown. Treasury Secretary Janet L. Yellen warned lawmakers on Tuesday of “catastrophic” consequences if Congress does not deal with the debt limit before Oct. 18.

The unease is apparent in stock performance the past four weeks. The S&P 500 is approaching a 4 percent drop for September, ending seven straight months of gains. The winning streak had lifted stocks more than 20 percent, as investors seemed to largely shrug off any bad news.

Bumpy moments have usually involved the Fed. Tuesday’s trading echoed the volatility of earlier this year, when a jump in rates roiled financial markets. That rise happened as traders worried that higher inflation might cause the Fed to increase rates sooner than officials had forecast.

“There’s no doubt that the equity market does not like higher rates — there’s just no debate about it,” Ralph Axel, director of U.S. Rates Strategy at Bank of America.

Lauren Goodwin, an economist at New York Life Investments, wrote in a note to clients that investors have begun seeking out safer investments while weighing concerns including the debt-ceiling fight and regulatory actions in China.

The Chinese government has shown signs of sharply shifting away from the policies that have guided its economy for much of the last decade, tightening regulation on subjects like online gaming and data sharing by tech companies. And Beijing has so far been reluctant to bail out the teetering Evergrande Group, a beleaguered residential developer with $300 billion in debt, another shift from typical policy.

But, Ms. Goodwin wrote, risks like that “should do little to impact the broader fundamental environment.” Instead, she said, the forces driving the market in the near future would remain those that have done so throughout the past 18 months: the spread of the virus, government spending, and decisions by the Federal Reserve.

“The path will depend heavily on our three highly uncertain drivers — the pandemic, monetary policy and fiscal policy,” she wrote.

While the slowdown of bond-buying will start sooner rather than later, the Fed’s main policy interest rate — its more powerful and traditional tool — remains near zero. And the Fed chair, Jerome H. Powell, and his colleagues have signaled that the central bank is a long way from raising interest rates because it wants to see the job market return to full strength before doing so.

“The test for raising interest rates is substantially higher,” Mr. Powell said at a Senate Banking Committee hearing on Tuesday. What the Fed wants to see, he said, is a “very strong” labor market: “The kind of thing that we did see before the pandemic arrived.”

Jeanna Smialek and Matt Phillips contributed reporting.