Billionaire Mike Bloomberg has pledged at least $ 100million (£ 78million) to help Democrat Joe Biden’s presidential campaign in Florida.
Of the battlefield states, Florida provides the largest share of the electoral college votes needed to win.
Mr Bloomberg has already spent $ 1 billion this year in an attempt to defeat Mr Biden to become the Democratic challenger.
Donald Trump has hinted that he will spend his own money in the state before early voting opens on September 24.
Both campaigns are keen to attract voters who can vote early by mail and set the tone for the final weeks of their campaign.
Postal voting is expected to double this year compared to 2016 due to the coronavirus pandemic encouraging voters to avoid the polling station.
Florida offers 29 of the 270 constituency voters needed to win the election. Only California (55) and Texas (38) offer more, but are considered safe Democratic and Republican territories respectively. President Trump won Florida in 2016, while Democrat Barack Obama took state in 2012.
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“Mike Bloomberg is determined to help defeat Trump, and it will happen in the battlefield states,” Kevin Sheekey, an adviser to Mr. Bloomberg, told Reuters.
In a tweet, Donald Trump responded by criticizing Mr Bloomberg’s own race to become the Democratic presidential candidate in which he reportedly spent $ 409 million (£ 313 million).
“I thought Mini Mike was done with Democratic politics,” he wrote of the former New York mayor. “Save NYC instead. ”
Earlier this week, when asked by reporters if he was considering spending his own money in Florida, Mr. Trump told reporters, “If I have to, I will. Either way, we have to win. ”
The president initially had a significant financial advantage over his rival, but Mr. Biden’s successful fundraising closed the gap.
A Cook Political Report poll this week suggested the competition in Florida was shrinking, with Mr. Trump improving his position in the race there.
Nationally, Biden leads the polls.
Mr Bloomberg endorsed Mr Biden after failing to win the Democratic nomination and spoke in prime time at the Democratic convention in August.