Mike Bloomberg Billionaire has pledged at least $100m (£78m) to help Democrat Joe Biden's presidential campaign in Florida.
Of the battleground states Florida offers the biggest share of the Electoral College votes needed to win.
Mr Bloomberg already spent $1bn this year trying to defeat Mr Biden to become the Democratic challenger.
Donald Trump has hinted he will spend his own money in the state ahead of early voting opening on 24 September.
Both campaigns are keen to appeal to voters who may be casting their votes early by post and set the tone of the final weeks of their campaigns.
Postal voting is expected to be double than seen in 2016 as the corona virus pandemic spurs voters to avoid polling stations.
Florida offers 29 of the 270 electoral college voters needed to win the election. Only California (55) and Texas (38) offer more but are seen as safe Democrat and Republican territory respectively. President Trump won Florida in 2016, while Democrat Barack Obama took the state in 2012.
"Mike Bloomberg has shown is committed to helping defeat Trump, in the battleground states," Kevin Sheekey, an adviser to Mr Bloomberg, told Reuters.
Earlier this week when asked by journalists if he was considering spending his own money in Florida, Mr Trump told reporters "If I have to, I will. Whatever it takes, we have to win."
The president initially had a significant financial advantage over his rival but Mr Biden's successful fundraising created has closed the gap.
A cook Political Report Poll ths week suggested that the contest in Florida is narrowing, with Mr Trump improving his standing in the race there. Nationally Mr Biden is leading in the polls.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.