At the end of January, a little after the first confirmed case of Covid-19 in the United States, the New York Stock Exchange hit an all-time high, and Donald Trump's biggest concern was the impact of Boeing's woes on the economy.

"The American Dream is back, bigger, better and stronger than ever before," the US president declared in Davos, already in campaign mode.

Fast-forward nine months. More than nine million confirmed infected with the coronavirus and close to 230,000 have died from Covid-19 -- making it the third leading cause of death for the year.

The numbers become even more grim when the hundred thousand or so additional deaths from misdiagnosed Covid-19 cases and indirect causes are added to the tally.

Despite an impressive rebound in the third quarter, millions of Americans lost their jobs, shattering the most reliable electoral argument of American presidents in search of a second term -- that of economic strength.

Trump's defeat at the polls next Tuesday is far from assured, and the Republican believes the electorate will punish his rival Joe Biden for failing to campaign extensively on the ground.

But his handling of the pandemic has certainly cost him votes.

Like that of Kimberly McLemore, a 56-year-old from St. Augustine, Florida who worked in jewelry before the pandemic hit.

McLemore, a lifelong Republican, said she thought Trump was doing a good job at the start, when he held daily briefings and seemed to listen to scientists, but she later realized he was not taking the crisis seriously.

"In good conscience, I cannot vote for this man," she told AFP, adding that both her parents, who are in their eighties, also voted for Biden, the first time they had voted for any Democrat.

"Vote like your life depended on it" is also how a narrator closes out an ad for The Lincoln Project, a collective of high-profile former Republicans on a mission to defeat Trump.

Thin margins

At rallies and on Twitter, Trump has accused the "fake news lamestream media" of focusing on "Covid, Covid, Covid" to hurt his reelection chances.

The virus has hindered his efforts since the start of the campaign.

In June, he held his first rally of the pandemic in an indoor arena in Tulsa, which somehow managed to be both an embarrassing disaster in terms of attendance but also likely a superspreading event.

Surveys for several months now have shown that Americans are judging their leader poorly when it comes to the virus.

Only 40 percent approve, according to a recent Gallup poll, compared to 60 percent in March.

These types of polls are too general to detect if there will be a so-called "Covid effect" on the election.

But on Friday, researchers behind a new analysis in the journal Science Advances said they found just that.

They drew on more 300,000 survey responses from the summers of 2019 and 2020, then tied that to local Covid-19 death rates, concluding the pandemic may have significantly damaged public support for Trump.

Specifically, people in counties which saw a doubling in the death rate in the month prior to the day they were surveyed were on average 0.14 percent less likely to vote Trump, and 0.28 percent less likely to vote for Republican congressional candidates.

That might not sound like a whole lot. But George Washington University's Christopher Warshaw told AFP it could still be significant.

"A lot of elections are determined by small margins. So even relatively small effects are sufficiently important," he said.

"It's very possible, based on our results, that Covid could be affecting the president and other members of his party, the two-party vote share by half a point or a point or more in some states and counties."

It still might be enough for Biden. In 2000, just a few hundred votes separated the winner and loser in the key state of Florida.

And four years ago, a meager 77,000 vote lead in three states (Pennsylvania, Michigan, Wisconsin) tipped the election in favor of Mr. Trump.