A stock market surge on Wednesday was again fueled not by concrete evidence of policy changes, but by off-the-cuff comments from President Trump and other officials, as investors latched onto scraps of information about tariffs, trade and other crucial issues that can shift from day to day.

Wall Street’s drastic swings this week — a sharp sell-off on Monday, followed by two big daily rallies — highlight how investors are swayed by the latest headlines amid the confusion and uncertainty about the White House’s intentions.

Wednesday’s trading was no different. Stocks surged to start the day, before paring back gains after Treasury Secretary Scott Bessent dismissed speculation that Mr. Trump would unilaterally lower tariffs imposed on Chinese goods. The S&P 500, which rose as much as 3 percent in early trading, settled to a gain of 1.7 percent for the day, extending the rally from the day before, when the index jumped 2.5 percent.

The initial enthusiasm came from Mr. Trump’s remark on Tuesday that he had “no intention” of firing the Federal Reserve chair, Jerome H. Powell, which helped lift markets on Wednesday. Days before, Mr. Trump had lashed out at Mr. Powell — “If I want him out, he’ll be out of there real fast,” he told reporters — which unnerved investors who see the Fed’s independence as critical to the health of the U.S. economy.

The seesawing moves this week echoed the whiplash from earlier this month when Mr. Trump paused many of his “reciprocal” tariffs hours after they were imposed, turning a steep market decline into a big gain.

Historically, many of the biggest single-day moves have occurred during down markets, said Steve Sosnick, the chief strategist at Interactive Brokers. Investors are always reluctant to miss out on a rally, he said.