The novel coronavirus-driven stock plunge that started less than a month ago has wiped out nearly 80% of the market gains that had accumulated since Donald Trump was elected president in November 2016. What's more, the S&P 500 is now up just 6% since Mr. Trump was inaugurated a little more than three years ago, in January 2017.

The latest flattening of what market mavens have called the Trump Bump came on Monday. The Dow dropped 2,997 points, falling nearly 13%, to 20,583 points. The broader S&P 500-stock market index slid 12%.

In all, stocks have now crashed nearly 30% since their peak on February 19. "Highest Stock Market In History, By Far!" Mr. Trump tweeted that day.

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The market is still up since Mr. Trump was elected, but not by much. Since February 19, investors in S&P 500 companies and in S&P 500 index funds — a widely held investment in many Americans' 401(k) retirement savings accounts —have seen nearly $8.3 trillion of their money vanish. As a result, the S&P 500 is now up $2 trillion since Mr. Trump was elected, or up just 15% over three-plus years. That translates into an average annual gain of less than 5%.

Mr. Trump has regularly cited the rise of the stock market throughout his presidency as evidence that his administration's pro-business policies have worked. In late 2017, Mr. Trump and Republican lawmakers pushed thought a massive tax cut that created a big boost in corporate profits that stoked stock prices but will cost the government roughly $2 trillion in lost revenue over 10 years.

It was never clear how directly Mr. Trump's policies were responsible for the rise of the stock market these past few years of an 11-year-long bull market. Similarly, it is unclear how much of the current fall is related to Mr. Trump's response to the coronavirus threat.

An exchange-traded fund called the Bridge Point GOP Stock Trader ETF, which trades under the ticker symbol MAGA, has fallen 33% since the February 19 peak, or slightly worse than the market in general has fared. According to Bridge Point's website, the MAGA fund invests in companies that will benefit from Republican policies.

Hal Lambert, who heads Bridge Point, said the fund had done worse than the rest of the market because it was weighted toward finance and energy stocks, two of the worst-hit sectors in the current downturn.

But Lambert said it's clear that Mr. Trump's policies had lifted the stock market.

"The drop in the stock market of the past three weeks has been based on a forced shutdown of the U.S. $20 trillion economy that will cause a global recession," said Lambert, who believes stocks would be down even more if Mr. Trump had moved quicker to stop the spread of the virus and shut down the economy sooner.

Lambert thinks a market rebound will begin soon. "I think stocks will higher by Election Day."