Investors have long been enamored with buybacks for that reason — even the announcement of such a program would often send a company’s stock price shooting higher — and companies have preferred them to paying out dividends. But critics of buybacks say they’re a poor use of company cash that could instead be invested in long-term growth and workers.

Companies in the S&P 500 stock index spent more than $2 trillion buying back their own stocks over the last three years, which helped solidify the status of corporations as the single largest source of demand for American stocks. But the recent slump has mostly wiped out the gains; more than $8 trillion in shareholder value has vanished over the past month.

Republicans who had pushed for the tax cuts said they would increase economic growth, prod companies to invest and raise wages for workers. But the results were mixed: Wages did rise, but growth in the gross domestic product didn’t accelerate meaningfully. And investment slumped, in part because of last year’s trade war with China. Democrats pointed to the boom as proof that the law was a giveaway to corporations and the wealthy — over 80 percent of household stock ownership is controlled by the wealthiest 10 percent of American households.

Now, with both the president souring on the practice and critics in Congress turning up the heat on buybacks as they hammer out the details of a stimulus bill, investors have grown leery, too.

In recent weeks, they’ve jettisoned stocks of companies that are the among the largest buyers of their own shares. From the market’s peak on Feb. 19 through the close of trading on Monday, the S&P 500 buyback index — which measures the performance of the top 100 companies with the highest buyback activity — fell 42.5 percent. That performance was worse than the regular S&P 500 index, which was down roughly 34 percent.

“It’s significantly underperformed,” said Ben Laidler, chief executive of Tower Hudson Research, a financial markets research firm in London. “The market is certainly worried about this for some of those stocks.”

Buybacks might shrink for the foreseeable future because companies would rather save their cash and don’t want to be singled out for criticism by politicians, analysts said. Already this year, announcements of new plans for buybacks are down more than 35 percent — in dollar terms — according to data from Birinyi Associates, a financial markets research group.