Former New York City mayor Michael Bloomberg has committed at least $US100 million to help Joe Biden's presidential campaign in the crucial battleground state of Florida.
Mr Bloomberg's late-stage infusion of cash reflects Democrats' concerns about the tight race in a state that is a priority for President Donald Trump.
A victory for Mr Biden in Florida, the largest of the perennial battleground states, would significantly complicate Mr Trump's path to reaching the 270 Electoral College votes needed to secure a second term.
Republicans, however, feel confident in their chances in the state, pointing to GOP wins in 2018 and stronger than expected turnout in 2016 as evidence the state is trending in their direction.
They have invested millions in Florida, focused on Latino outreach and boosting their field operation, and the state's size and diversity makes campaigns there expensive.
In a sign the planned investment put Mr Trump on alert, the president tweeted out his disdain for Mr Bloomberg on Sunday morning, referencing the attacks the businessman received at a Democratic primary debate in February from Massachusetts Senator Elizabeth Warren.
He tweeted: "I thought Mini Mike was through with Democrat politics after spending almost 2 Billion Dollars, and then giving the worst and most inept Debate Performance in the history of Presidential Politics. Pocahontas ended his political career on first question, OVER! Save NYC instead."
Billionaire Mr Bloomberg launched his own campaign for the Democratic nomination late last year amid worries about Mr Biden's strengths.
Despite spending a billion dollars on his campaign, Mr Bloomberg struggled and dropped out in March, quickly endorsing Mr Biden.
One of the world's wealthiest men with a net worth estimated to exceed $A82 billion, Mr Bloomberg exited the presidential race pledging to spend "whatever it takes" to defeat Mr Trump.
He has already invested millions to support Democrats up and down the ballot.
Mr Bloomberg transferred $A25 million from his presidential campaign to the Democratic National Committee and transferred its offices in six key swing states to the local Democratic parties there.
Former New York City Mayor Mike Bloomberg Source: Michael Brochstein/Sipa USA
The businessman has contributed $A670.000 to Voto Latino to help register Latino voters, $A2.7 million dollars to the group Collective Future to help register African American voters and more than two million to Swing Left, a group focused on electing Democrats in swing districts.
His new spending is intended to boost Mr Biden before the start of early voting in Florida, which begins on 24 September.
An adviser for Mr Bloomberg said much of the money will go to television and digital advertising.
