Media mogul Michael Bloomberg has pledged to inject at least $US100 million ($A137 million) into Joe Biden’s presidential campaign in Florida.

The former New York City mayor hopes his cash will help tip the state in favour of Mr Biden, as Donald Trump’s advisors believe the US President must win Florida to get re-elected.

A recent national poll by Monmouth University indicated that Mr Biden leads Mr Trump by nine percentage points among registered voters and seven percentage points among likely voters.

But Mr Trump has maintained his popularity in Florida, where he won by about 112,000 votes in 2016.

Given it is the largest of the perennial battleground states, a victory for Mr Biden in Florida would significantly complicate Mr Trump’s path to securing a second term.

In response to Mr Bloomberg’s cash infusion, the president tweeted about the attacks the businessman received at a Democratic primary debate in February from Massachusetts Senator Elizabeth Warren.

Mr Trump wrote: “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,” he continued.

In the highly competitive world of high tech, how come nobody has ever challenged Mini Mike Bloomberg and his very average, & supposedly outdated, “stuff” that he sells to so many? Did he use his position as Mayor of New York to create what is now a monopoly? Just askin’?

— Donald J. Trump (@realDonaldTrump) September 13, 2020

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.

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 $US60 billion ($A82 billion), Mr Bloomberg exited the presidential race pledging to spend “whatever it takes” to defeat Mr Trump.

NATIONAL POLL: #2020GeneralElection matchup for president:
REGISTERED VOTERS:
51% @JoeBiden
42% @realDonaldTrump
2% Jo @Jorgensen4POTUS
2% @HowieHawkins

LIKELY VOTERS:
51% Biden
44% Trump
1% Jorgensen
1% Hawkinshttps://t.co/jcAyZXjJPG pic.twitter.com/qIDUpeQHBt

— MonmouthPoll (@MonmouthPoll) September 10, 2020

He has already invested millions to support Democrats up and down the ballot.

Mr Bloomberg transferred 18 million dollars ($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.

The businessman has contributed $US500,000 ($A670.000) to Voto Latino to help register Latino voters, $US2 million ($A2.7 million) dollars to the group Collective Future to help register African American voters and 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 September 24.

An adviser for Mr Bloomberg said much of the money will go to television and digital advertising.

-with AAP