US Democratic presidential candidate Joe Biden has criticised Donald Trump's handling of Covid-19, while courting elderly voters in the key battleground state of Florida.

He told them that the president saw seniors, who have been more at risk in the pandemic, as "expendable".

There are sharp policy differences between the two candidates on Covid-19.

In Pennsylvania, Mr Trump told thousands of supporters he felt like "Superman" after his Covid treatment.

The president tested positive for the virus on 1 October, spent three nights in hospital and was cleared by doctors to return to the campaign trail at the weekend, holding his first rally in Florida on Monday.

Battleground states like Florida and Pennsylvania are crucial for gathering the 270 electoral college votes needed to win the presidential election, which is not determined by a simple count of votes nationwide.

Opinion polls suggest Mr Biden has a 10-point advantage over Mr Trump nationally, but his lead in some key states is narrower. In Florida, the Democrat is 3.7 percentage points ahead, according to an average of polls collated by Real Clear Politics.

Mr Trump narrowly won Florida in 2016 in a result buoyed by senior voters. But the latest polls suggest a shift away from the Republican among them this time around.

What did Biden say in Florida?

The Democratic candidate spoke to a group of people at a community centre for seniors in southern Florida, with social distancing measures in place.

The event was in stark contrast to the president's mass rally on Monday in Florida.

Mr Biden accused the president of dismissing the threat that coronavirus posed to senior citizens.

"You're expendable, you're forgettable, you're virtually nobody. That's how he sees seniors. That's how he sees you," said Mr Biden.

The "only senior Donald Trump seems to care about" is himself, he added.

Mr Biden also criticised the president for holding "super-spreader parties with Republicans hugging each other without concern of the consequences", while senior citizens couldn't see their grandchildren. A recent White House event for the Supreme Court nominee led to several attendees testing positive for Covid.

Introducing the Democratic candidate at the Florida event, congresswoman Debbie Wasserman Schultz said it was voters aged 65 or over who would "swing elections in the Sunshine State".

Joe Biden, 77, and Donald Trump, 74, are the two oldest candidates to contest a US presidential election.

But Mr Trump has regularly mocked Mr Biden as a senior citizen who lacks energy and is "sleepy". The president on Tuesday tweeted a doctored image of Mr Biden as a wheelchair-user and the words "Biden for Resident", implying a nursing home.

Opponents took to social media to say the tweet was disrespecting the elderly.

Why is Florida so important?

Despite trailing nationally, close races in crucial states like Florida mean Mr Trump may win re-election.

Under the electoral college system a candidate can win the most votes nationally but still lose, as happened to Hillary Clinton in 2016.

Each state has a set number of electoral college votes based on its number of Congress representatives, so the hugely populous state of California has 55, while Hawaii has four. Florida has 29.

In all but two states, the winning candidate gets all the state's electoral college votes and needs 270 of the 538 to win the presidency.

This year's election has seen a number of issues that mark it as different.

Opinion polling suggests as many as nine out of 10 voters have already decided which way to vote, meaning there is limited leeway for the candidates to change minds.

The number of people who have already cast ballots in early voting is closing in on 10 million, an unprecedented number.

Postal voting will also be much higher, mainly due to Covid-19. Mr Trump has used this to regularly cite the possibility of election fraud, although the Election Commission says there is no basis for fearing that.

What did Trump say in Pennsylvania?

He appeared on an airport tarmac in Johnstown before a packed crowd. The supporters were divided on mask-wearing. Some chose to don a MAGA (Make America Great Again) inscribed face-covering.

Mr Trump again railed against his Democratic challenger, saying Joe Biden was "not a nice guy".

"He's shot, folks. I hate to tell you, he's shot... can you imagine if you lose to a guy like this? It's unbelievable."

Mr Biden's campaign has made much of his childhood in the state, but Mr Trump told the rally: "They say he was born in Scranton. But he left. He left. He abandoned you. Give me a break."

The president also attempted to woo the key voting demographic of suburban women, many of whom, opinion polls suggest, view Mr Trump unfavourably.

"So can I ask you to do me a favour?" the president said. "Suburban women, will you please like me? I saved your damn neighbourhood." Mr Trump has projected himself as a defender of suburban America amid a wave of street protests this year.

Mr Trump said of his coronavirus treatment: "All I know is I took something, whatever the hell it was, I felt good very quickly... I felt like Superman."

The president has generally expressed scepticism about measures such as masks and lockdowns to combat the spread of Covid-19, which has killed more than 215,000 people in the US.

Mr Trump has other rallies planned this week in Iowa, North Carolina, Georgia and Florida again.