During his first term, President Trump regularly took credit for a booming stock market, citing soaring share prices as a measure of his success in office.

And after his election victory in November, he pointed to the market rise as a sign of optimism.

But since his inauguration in January, Mr. Trump has been relatively muted about stocks, even after the S&P 500 hit a record on Feb. 19.

The S&P 500 has since fallen almost every day, and the index is now lower than it was when Mr. Trump took office on Jan. 20.

Other indexes, including those more closely tied to the ebb and flow of the economy, have also fallen.

On Friday, the S&P 500 surged higher late in the day but remained lower for the week, its second consecutive week of losses for the first time since October. Other bullish reflections of Mr. Trump’s election have also faded, with Bitcoin tumbling roughly 20 percent over the past month.

While it is only less than two months into the new administration, the market is showing signs of weakness, as investors have become increasingly nervous about an impending sell-off. Consumer sentiment is souring and investors are growing weary over the cacophony of policy proposals emanating from Washington.

“The tariff rhetoric has become daily and extreme, sentiment is awful and trading is on edge,” said Andrew Brenner, head of international fixed income at National Alliance Securities.

Today’s market is fundamentally different than it was when Mr. Trump first entered the White House in early 2017.

For the two years preceding Mr. Trump’s first term, the S&P 500 had bumbled along. Stock and bond markets had grappled with the fallout of a domestic energy crisis as debt-laden oil companies had started going bankrupt.

Interest rates were roughly 4 percentage points lower than they are now, as the Federal Reserve tried to gently heat up the economy and revive tepid growth. As Mr. Trump unleashed tax cuts and promoted a pro-growth agenda, stocks were primed to rise.

“Highest stock market EVER,’’ Mr. Trump wrote in a social media post in July 2017.

But as Mr. Trump entered his second term, the stock market had already surged, with valuations at historic highs. The S&P 500 had risen more than 20 percent for two consecutive years — in 2023 and 2024 — for the first time since 1996. That means reaching new heights could prove harder, especially as the engine of the recent rally, big tech stocks, begins to sputter.

The market’s composition has also shifted since Mr. Trump’s first term.

The huge companies driving the A.I. boom have become so large that even small changes in their individual stock prices can move the entire S&P 500 index. That makes for a sometimes volatile market.

These stocks, known as the Magnificent Seven, together now account for roughly a third of the entire index by market value.

As these stocks’ share prices rose, they helped to lift the rest of the market. The risk is, as their prices fall, they become an anchor weighing the market down.

With competition among A.I pioneers intensifying, some of these marquee stocks have already lost their luster. Nvidia, which makes chips for A.I. companies, has fallen almost 10 percent since Mr. Trump’s inauguration. Of the S&P 500’s 11 sectors, tech is one of only two sectors that have declined so far this year

These moves have so far been largely unattributed to the new administration, illustrating the challenge facing Mr. Trump — and any president — seeking to engineer a buoyant stock market, especially if big tech continues to languish.

At a conference in Miami on Feb. 19, the day that the S&P 500 notched a record high, Mr. Trump said: “I think the stock market’s going to be great.”

In his speech, he exaggerated the gain made by the Dow Jones Industrial Average since the election, saying it had risen 10 percent, when it had risen less than 7 percent. The S&P 500 was up as much at 6.25 percent from the election to Feb. 19, but is now only 3 percent higher since Nov. 5.

Investors are notably nervous. While few foresee a full-blown recession, according to Bank of America’s latest survey of fund managers, many are wary of the market’s uncertain direction, especially given the potential for reciprocal tariffs to spark a trade war, upend the Federal Reserve’s fight with inflation and crimp economic growth.

Almost 90 percent of survey respondents said that stocks are overvalued. The CBOE Skew Index, which measures how much investors are girding for a sell-off by tracking trades in options markets that would buffer against a sudden plunge in the value of the S&P 500, reached its highest level ever on Feb. 18, the day before that index hit its record high.

It suggests that investors are nervous that the market could soon tumble. And that could be why the market is no longer the barometer of success that Mr. Trump once claimed it was.