Billionaire Mike Bloomberg 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 this year compared to 2016 due to the coronavirus pandemic encouraging voters to avoid the polling station.
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 is committed to helping defeat Trump, and that is going to happen in the battleground states,” Kevin Sheekey, an adviser to Mr Bloomberg, told Reuters.
In a tweet, Donald Trump reacted by criticising Mr Bloomberg’s own run to become the Democratic candidate for president in which he reportedly spent $409m (£313m).
“I thought Mini Mike was through with Democrat politics,” he wrote about the former mayor of New York City. “Save NYC instead.”
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 has closed the gap.
A Cook Political Report poll this week suggested that 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.