The stock market has plunged since President Trump took office for a second time, a stark contrast to the early days of his first presidency. The S&P 500 index has fallen sharply since hitting a record high on Feb. 19.

The index fell into a so-called correction on Thursday. Corrections, a Wall Street term referring to a 10 percent decline from an index’s last peak, are relatively rare yet symbolically worrisome milestones for the markets. Other major indexes, including the Nasdaq and the Russell 2000, had already slid into correction territory before Thursday.

Stock prices were once Mr. Trump’s favorite proxy for political success. During his first term, he continually took credit for a booming stock market.

“Whether you like the stock market or not, it’s a leading indicator — the all-time leading indicator,” he told reporters in October 2020.

This time around, investors have been shaken by the Trump administration’s messaging on tariffs. What at first seemed to some like a talking point or a negotiation tactic has started to take effect, in fits and starts, as policy.

Mr. Trump has reiterated his commitment to sweeping tariffs on America’s largest trading partners but has been uneven about imposing them, announcing new levies on some products while delaying or abruptly calling off others.

The market turmoil of the past few weeks is a reflection of the uncertainty surrounding Mr. Trump’s policies, and their potential effects on the broader economy. The worry is that consumers may be hesitant to spend, and businesses to invest, in the face of the uncertainty, driving the economy into a downturn.

And Mr. Trump’s immigration policies and firings of federal employees are also looming.

The Trump administration has seemingly acknowledged that its economic policies could result in near-term pain, while emphasizing their goal of promoting long-term job growth. Asked if he expected a recession this year in an interview that aired on Sunday, Mr. Trump declined to rule out the possibility.

“I hate to predict things like that,” he responded. “There is a period of transition, because what we’re doing is very big.”

The president inherited a very different market from the one he took on when he first entered the White House. After two years of tepid growth in the aftermath of a domestic energy crisis, stocks were primed to rise in 2017 on the heels of Mr. Trump’s pro-growth agenda. As his second term began, the stock market had already reached record highs. That meant reaching new heights could prove harder.

Mr. Trump recently hinted that the stock market might no longer be the barometer for success it once was.

“Markets are going to go up, and they’re going to go down,” he told reporters this week. “But you know what? We have to rebuild our country.”

Joe Rennison contributed reporting.