Now polls show Biden leading in all three, albeit narrowly in Iowa.
In Minnesota, Biden accused Trump of 'giving up' in the fight against the virus.
Zuckerberg expressed his concern while describing safeguards against misinformation and voter suppression.
Trump trails Biden in national opinion polls.
US president continues to ridicule masks and mask-wearing as he insists the county has turned the corner on the virus
Trump and Biden concentrated their efforts on the key state of Florida, where the president sought to raise enthusiasm for an economic recovery to counter polls.
Curfew imposed in Philadelphia after violent protests erupt following the gunning down of Walter Wallace Jr by police
Meanwhile, Biden says it will 'take a lot of hard work' to end the Covid-19 pandemic.
Trump lost the popular vote to Hillary Clinton in 2016 by nearly three million votes and is likely to lose it to Biden also.
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