President Donald Trump has returned to the campaign trail less than two weeks after testing positive for coronavirus.
He told thousands of supporters, many not wearing masks, that he could give them "a big fat kiss", at a rally in the battleground state of Florida.
His rival, Joe Biden, speaking in Ohio, accused the president of "reckless behaviour" since his diagnosis.
The two presidential candidates are scrambling to secure votes with three weeks until the 3 November election.
Opinion polls suggests Mr Biden has a 10-point lead over Mr Trump nationally. However his lead in some key states is narrower - as is the case in Florida, where the Democrat is 3.7 percentage points ahead, according to an average of polls collated by Real Clear Politics.
Battlegrounds like Florida and Ohio are crucial for gathering the 270 electoral college votes needed to win the White House, which is not determined by a simple count of votes nationwide.
Mr Trump, 74, tested positive for Covid-19 some 12 days ago, and was admitted to hospital a day later.
But on Sunday his personal doctor said he was no longer a Covid transmission risk to others and disclosed on Monday that his most recent tests were negative over consecutive days, although he did not give the dates.
Top US government scientist Anthony Fauci said holding big campaign rallies was "asking for trouble" as many states are battling an increase in coronavirus cases.
Trump faithful come out to see their man
At the scene - Nomia Iqbal, BBC News, Sanford, Florida
The usual chants of "Four more years!" were heard as hundreds queued to enter the open-air space where the president made his appearance.
People have been getting their temperature checked and were offered face masks. Those who covered up didn't think it was too soon for the president to be out and about. They said they admired him for it. One man here told me he was originally from New York and had made Florida his home - just like "his hero" Donald Trump.
The Trump campaign team is throwing everything at Florida - a loss here makes his return to the White House near impossible.
Vice-President Mike Pence recently visited "The Villages", a famous retirement community, to court the elderly vote.
But though the president and some of his supporters here seem relaxed about coronavirus, it has hit Florida hard, with more than 15,000 deaths.
Joe Biden, who arrives in the state on Tuesday, is focusing on the administration's handling of the pandemic and plans to carry out a quieter but vigorous campaign in key suburban areas, which he hopes will help flip the state blue.
What did Trump say at the rally?
In his first stump appearance following his Covid-19 diagnosis and recovery, a re-invigorated Mr Trump returned to his campaign's familiar themes and lines of attack against Mr Biden.
He touted stock market growth, the establishment of the US Space Force and his successful confirmation of two conservative Supreme Court justices to the bench - with a third nominee, Judge Amy Coney Barrett, before the Senate this week - as major achievements.
In front of a crowd of several thousand he denounced plans to prolong Covid-related shutdowns supported by Democrats and sought to question Mr Biden's mental acuity.
Referring to his own recovery from Covid-19, he said at one point: "They say I'm immune - I feel so powerful. I'll walk in there and kiss everyone. I'll kiss the guys and the beautiful women, I'll give you a big, fat kiss."
Though reinfections of the virus remain rare scientists are still questioning how long any potential immunity to the virus lasts.
Few people in the crowd were seen wearing masks or adhering to public health guidelines to keep at least 6ft (2m) apart to reduce the risk of transmitting Covid-19.
Speaking to CNN, Dr Fauci said it was not the time to be holding full campaign rallies as the country battles the pandemic.
"We know that that is asking for trouble when you do that," he said.
"Now is even more so a worse time to do that, because when you look at what's going on in the United States, it's really very troublesome," he added, referring to increases in test positivity rates in a number of states.