Washington (AFP) - Already shaken by multiple crises, the US presidential race tumbled deeper into turmoil with Donald Trump's Covid-19 diagnosis, leaving the campaign in unprecedented flux four weeks before the November 3 election.

Here is a look at the state of the race between Trump and his Democratic rival, former vice president Joe Biden, and what to expect in the weeks before Election Day.

Trump sick, sidelined

Uncertainty has surrounded the 74-year-old commander in chief's coronavirus illness.

His doctors have offered conflicting messages, and did not fully explain why the president was administered an experimental antibody cocktail if his progress was satisfactory.

A top aide nevertheless said early Monday he was "optimistic" that Trump could return from Walter Reed military medical center to the White House later in the day.

One thing is certain: testing positive has sidelined Trump from the campaign trail at a crucial moment. Trump's trademark boisterous rallies are on hold, denying him access to voters in swing states like Florida and Pennsylvania that he must hold if he is to win re-election.

Biden campaigns, 'safely'

Trump's sequester is not hampering Biden's schedule. He paused anti-Trump ads for a day but not his in-person campaigning in battleground states.

"Since the start of the pandemic, our campaign has led by example and prioritized the health and safety of our supporters, our staff and the public in everything that we do," Biden national press secretary TJ Ducklo told AFP Monday.

"We take extraordinary measures to ensure we are campaigning safely" and that Biden and those around him are "protected," Ducklo added.

On Monday Biden -- who has repeatedly tested negative since Friday -- headed to Florida to court Hispanic voters, a key constituency in the Sunshine State, then attends a live televised town hall. He flies to Arizona on Thursday.

Biden has been meticulous about wearing masks in public, modeling behavior that government health experts say everyday Americans should be conducting, a contrast with Trump who has largely shunned mask-wearing.

Debate over debates

With Trump symptomatic, the remaining debate schedule is up in the air, although no decision has been announced by the White House.

Asked whether it would be safe for him and Trump to square off next week, Biden, 77, told reporters Monday that "I'll do whatever the experts say."

But he added: "I think we should be very cautious."

After the first debate devolved into an indoor shoutfest last Tuesday in Cleveland, concern has emerged about whether the remaining debates should be held outside, with masks mandatory, held virtually or scrapped altogether.

A town-hall-style event is set for October 15 in Miami, but will a 74-year-old president recovering from Covid-19 be ready to take voter questions for 90 minutes less than two weeks after his announced diagnosis?

"I can't see any reason why he wouldn't, and there's no reason to delay" the debates, Trump lawyer Rudy Giuliani, the former mayor of New York, told ABC.

"They're enormously valuable to the American people, and I think he'll make every effort to make it."

But the focus now is on the vice presidential debate, set for Wednesday in Utah, and which looms large with Trump sidelined.

Vice President Mike Pence and Biden's running mate Kamala Harris have both tested negative, and the clash will be the primary opportunity for the deputies to showcase their political skills.

Polls favor Biden

Most signs point to an uphill battle for Trump who has trailed in polls over several months. The RealClearPolitics poll average shows Biden leading by 8.3 percentage points.

The latest major survey, by NBC News and the Wall Street Journal, has Biden leading by a healthy 53-39 margin. The poll was conducted after the first debate, but before Trump's diagnosis.

The race appears tighter in the all-important battlegrounds. But Biden is ahead in several key states that Trump won in 2016, including Florida (Biden up by 2.0 points), Pennsylvania (6.5), Michigan (5.2), Wisconsin (5.5) and Ohio (1.2).

State of vote

With Americans eager to vote by mail to diminish the Covid threat, early voting numbers have soared.

More than 3.5 million ballots have already been cast, according to the US Elections Project.

So even if Trump's diagnosis tilts voters one way or the other, for many Americans it is too late.

The election consequences of Trump's illness remain unclear, but one could look at the "Boris bounce" as a sign that a rally-round-the-flag effect can occur in times of personal crisis.

British Prime Minister Boris Johnson's approval rating spiked after he contracted Covid-19 and was hospitalized in early April.