United Airlines employees work at ticket counters at Chicago's O'Hare International Airport on Oct. 14. United and other airline stocks have been hard-hit by the pandemic economic slowdown.

United Airlines employees work at ticket counters at Chicago's O'Hare International Airport on Oct. 14. United and other airline stocks have been hard-hit by the pandemic economic slowdown. / AP

Updated at 5 p.m. ET

Stocks on Monday posted their worst day since early September amid a surge in coronavirus cases in the United States and Europe and declining optimism about another U.S. pandemic relief bill.

The Dow Jones Industrial Average ended the day down 650 points, or 2.3%, posting its biggest decline since Sept. 3. The other major indexes were also down, though not as much.

The declines happened amid reports that coronavirus cases have spiked in the United States, with more than 80,000 new infections reported on Friday and Saturday, while France and Italy registered a record number of new cases.

"Coronavirus worries are back," said Lindsey Bell, chief investment strategist at Ally Invest. In addition to new cases reaching records, she said, "the timing of a vaccine continues to be crucial, but elusive, adding to concerns about economic prospects."

Companies slammed by the pandemic, such as airlines and travel companies, were among those hardest-hit, with United Airlines ending down 7%.

"There continue to be wide swaths of the U.S. economy which simply cannot get back to normal in a worsening pandemic, including travel, leisure, entertainment, restaurants and bricks-and-mortar retailing," said David Kelly, chief global strategist at J.P. Morgan Funds.

But declines took place across the board. Energy companies, which see their profits fall when the economy slows, were down.

Meanwhile, the odds that Congress and the White House can agree on another stimulus bill are fading.

While House Speaker Nancy Pelosi and Treasury Secretary Steve Mnuchin spoke several times about a bill last week, White House officials say significant differences remain between the two sides.

"The failure of Congress and the administration to agree on a further stimulus package before the election will likely result in more small-business closures and state and local layoffs in the fourth quarter, along with some slowing in consumer spending on the basics," Kelly said.

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Tags: COVID-19  stocks  coronavirus  Stock market  Series: The Coronavirus Crisis