Former New York mayor and candidate for the Democratic presidential nomination Mike Bloomberg has said he will spend at least $100m to support Joe Biden in Florida.
Mr Bloomberg already spent $1bn this year trying to defeat Mr Biden to become the Democratic challenger.
Mr Bloomberg endorsed Mr Biden after failing to win the Democratic nomination and spoke in a prime-time slot at the Democratic convention in August.
News of Bloomberg’s intervention in Florida prompted a characteristic response from the president, who was campaigning in Nevada.
Donald Trump has hinted he will spend his own money in the state ahead of early voting opening on 24 September.
“I thought Mini Mike was through with Democrat politics after spending almost $2bn,” Trump tweeted.
The president initially had a significant financial advantage over his rival but Mr Biden's successful fundraising has closed the gap.
Kevin Sheekey, an adviser to Mr Bloomberg, told Reuters: "Mike Bloomberg is committed to helping defeat Trump, and that is going to happen in the battleground states."
Shekeey said that Bloomberg believes that by investing in Florida it will allow campaign resources and other Democratic resources to be used in other states, in particular Pennsylvania.
A Bloomberg aide said the money would be used to help bring people to the polls who support Biden and communicating with Latino voters, in particular.
Florida offers 29 electoral college votes, vital in a close race. No Republican has won the White House without it since 1924. Infamously, George W Bush, the last two-term Republican president, won Florida against Al Gore in 2000 after multiple recounts and with the intervention of the US supreme court.