The Trump Bump is becoming the Trump Slump.

In the first year of Donald J. Trump’s presidency, ebullient investors propelled stock markets to one record high after another. And Mr. Trump was the bull in chief, celebrating the record-breaking march as validation of his economic policies.

Those days are done.

Even after a fast start to 2018, stock markets finished the first quarter down for the year — the first quarterly decline since 2015. It suggested that a period of calm and steadily rising markets had given way to a turbulent new era with a bearish bent.

The plunge continued Monday, with the Standard & Poor’s 500-stock index sinking 2.2 percent. Investors jettisoned shares of financial, technology and many other businesses, spooked at least in part by a tweet from Mr. Trump aimed at one of the country’s biggest companies: Amazon.

Asian and European markets were down more modestly in midday Tuesday trading.

Monday’s decline left stocks down more than 4 percent so far in 2018. They are now down more than 10 percent from their peak in late January. That means the market has entered a so-called correction — a term used to indicate that the downward trend is more severe and lasting than simply a few days of bearish trading.