Stock markets slumped on Monday, the first full day of trading after President Trump’s efforts at health care reform fell short, an apparent sign to investors that hopes of tax reform and financial stimulus may be in doubt.

The major market indicators fell to their lowest levels since early February on Monday. After dropping about 1 percent, the benchmark Standard & Poor’s 500-stock index was down 0.5 percent in midmorning trading.

Buoyed by the promise of both houses of Congress and the White House being in Republican control, the stock market had been on a monthslong rally as investors anticipated measures on regulations and taxes that would be friendly to business, coupled with hefty spending on infrastructure.

But the collapse on Friday of a health care bill — a legislative priority that not only dominated much of President Trump’s first two months in office, but also much of his election campaign — has cast uncertainty on the other parts of his agenda. Congressional Republicans have already started discussing scaling back ambitious tax reform plans.

“The U.S. market had already started to weaken over the last month over concerns about the U.S. administration, and the reform agenda that they had,” said Graham Secker, a London-based equities strategist at Morgan Stanley. “It crystallized those concerns on Friday.”

“Arguably, people were too complacent on the political uncertainty,” he added.

Stocks of banks and other financial companies led the downturn on Monday. The Dow Jones industrial average was down 0.6 percent, while the Nasdaq composite was down 0.4 percent.

Stocks began their rise after Mr. Trump’s election in November, with the S.&P. 500 rising 12 percent through to a peak of 2,396 on March 1. But as concerns have risen that he may be unable to fulfill a wide-reaching plan for reforms, investors’s enthusiasm has begun to dampen.