Ed Yardeni, the renowned Wall Street bull, has suggested that a bear market may have already begun. "Anything is possible in Trump World," he said.

Yardeni wrote last Sunday, "We can't rule out the possibility that a bear market started on Feb. 20, the day after the S&P 500 rose to a record high."

Yardeni's remarks are noteworthy because he has previously been bullish, often ranking among Wall Street's most optimistic strategists. As recently as February, he stated that the U.S. economy might avoid a recession for the next decade. In January, he declared that investors were in the midst of the so-called Roaring 2020s.

The strategist's latest report comes after the major U.S. stock indices experienced their worst week of the year. The sell-off continued in early trading this week.

On Monday, the Nasdaq Composite entered correction territory, down more than 13% from its December all-time high. The S&P 500, down more than 8% from its February 19 peak, is also rapidly approaching correction territory.

A bear market is defined as a broad decline of 20% or more in stock prices, which can last for months or even years. According to Yardeni, this means the recent downturn could worsen. However, for investors with a shopping list, the market decline could also provide buying opportunities for some oversold stocks.

"It could be like the 'flash crashes' that occurred during 1962 and 1987. It could happen quickly and reverse just as quickly,"  Yardeni said. Therefore, "the selloff could provide buying opportunities, especially in overvalued names that are now less so."

His shift in perspective is due to the unpredictability of Trump's trade policies and early signs of economic weakness, which have heightened concerns about a potential recession. A recession is defined as two consecutive quarters of economic contraction.

"The problem is that the Trump administration's rapid-fire policy initiatives are stress-testing the economy, which has been remarkably resilient so far, and raising fears of a recession,"  Yardeni added.