President Trump inherited an economy that was, by most conventional measures, firing on all cylinders. Wages, consumer spending and corporate profits were rising. Unemployment was low. The inflation rate, though higher than normal, was falling.

Just weeks into Mr. Trump’s term, the outlook is gloomier. Measures of business and consumer confidence have plunged. The stock market has been on a roller-coaster ride. Layoffs are picking up, according to some data. And forecasters are cutting their estimates for economic growth this year, with some even predicting that the U.S. gross domestic product could shrink in the first quarter.

Some commentators have gone further, arguing that the economy could be headed for a recession, a sharp rebound in inflation or even the dreaded combination of the two, “stagflation.” Most economists consider that unlikely, saying growth is more likely to slow than to give way to a decline.

Still, the sudden deterioration in the outlook is striking, especially because it is almost entirely a result of Mr. Trump’s policies and the resulting uncertainty. Tariffs, and the inevitable retaliation from trading partners, will increase prices and slow down growth. Federal job cuts will push up unemployment, and could lead government employees and contractors to pull back on spending while they wait to learn their fate. Deportations could drive up costs for industries like construction and hospitality that depend on immigrant labor.

“If the economy was starting out in quite good shape, it’s probably in less good shape after what we’ve seen the last few weeks,” said Donald Rissmiller, chief economist at Strategas, a research firm.

A Strong Foundation

The U.S. economy has repeatedly shown its resilience in recent years, and there are parts of Mr. Trump’s agenda that could foster growth. Business groups have responded enthusiastically to Republican plans to cut taxes and reduce regulation. A streamlined government could, in theory, make the overall economy more productive.