WASHINGTON — Democratic presidential candidates have seized on President Trump’s response to the spreading global coronavirus outbreak, and the growing threat it poses to America’s record-long economic expansion, to attack the president on what has been his greatest strength with voters: the economy.

Until last week, the candidates had largely attacked Mr. Trump’s economic management on inequality grounds, at a time when growth has been steady and unemployment has sunk to a half-century low. But they have begun to attack his stewardship more directly after fears over the effects of the virus dealt stock markets their worst week since 2008 and forced Federal Reserve officials to reassure investors that they were considering interest rate cuts to combat a potential growth slowdown.

Two candidates in desperate need of delegates on Super Tuesday, Senator Elizabeth Warren of Massachusetts and former Mayor Michael R. Bloomberg of New York, revamped their stump speeches in recent days to aggressively attack Mr. Trump’s handling of the issue and portray themselves as the type of president the United States needs to endure a potential economic and public health crisis.

Mr. Trump has played down the virus, insisting several times last week that it might not spread any further in the United States, and the economic threats from it. And he has lashed out at Democrats, saying they were the ones spooking investors.