President Trump made a lot of promises on the campaign trail last year. Investors and business leaders enthusiastically cheered some, like lower taxes and relaxed regulation, and expressed wariness about others, like tariffs and reduced immigration.

But when Mr. Trump won the election, there was little sign of that ambivalence: Stock prices soared, as did measures of business optimism.

Investors at the time offered a simple explanation: They believed Mr. Trump, backed by a Republican-controlled Congress, would follow through on the parts of his agenda that they liked and scale back the more disruptive policies like tariffs if financial markets started to get spooked.

It is increasingly clear they were wrong.

In his first weeks in office, Mr. Trump has made tariffs the central focus of his economic policy, promising, and at times imposing, steep penalties on allies as well as adversaries. He has threatened to curb subsidies that businesses had come to rely on. And he has empowered Elon Musk’s efforts to slash the federal bureaucracy, potentially putting tens of thousands of federal workers out of jobs and cutting off billions of dollars in government grants and contracts.

Most surprising, at least to the optimists on Wall Street: Mr. Trump has so far been undeterred by signs of cracks in the economy or by plunging stock prices.

“The idea that the administration is going to be held back by a self-imposed market constraint should be discounted,” said Joe Brusuelas, chief economist at the accounting firm RSM.