“In order to make America great and glorious again. I am tonight announcing my candidacy for president of the United States.”

With those words former US president Donald Trump confirmed the worst-kept secret in American politics – that he plans to run again in 2024.

According to multiple reports from the US, Mr Trump filed the paperwork to establish his candidacy for the presidency on Wednesday afternoon (AEDT). Just hours later he appeared on stage in Florida to confirm the announcement to an adoring crowd at Mar-a-Lago, his waterfront estate.

“Ladies and gentleman, America’s comeback starts right now,” he said to applause and cheers of “USA, USA”.

“Three years ago when I left office … our nation was at the pinnacle of power and prestige. Striding into the future, confident and so strong,” he said.

“Everybody was thriving like never before.”

Mr Trump made the announcement after rattling off a litany of supposed achievements of his administration. His meandering speech at times veered into conspiracy theories and hit on many of the beats and topics popular with his followers.

Mr Trump painted a picture of a dystopian America under President Joe Biden and prophesied the decline of the nation under Mr Biden’s continued presidency.

“I am running because I believe the world has not yet seen the true glory of what this nation can be. We have not reached that pinnacle, believe it or not,” he told cheering fans.

Mr Trump hopes to box out potential Republican rivals and return his false claims of election fraud to the centre of US politics. Wednesday’s announcement follows a disappointing showing in last week’s midterm congressional elections that many Republicans blame on him.

Hours before the speech, supporters began filling a ballroom decorated with several chandeliers and lined with dozens of American flags.

• Inside Donald Trump’s ‘very big announcement’

The unusually early launch might well be aimed at fending off potential challengers for the party’s nomination in 2024, including rising star Florida Governor Ron DeSantis, 44, and Mr Trump’s own former vice president, Mike Pence, 63.

It came as Republicans closed in on the 218 seats they need to take a majority in the 435-seat House of Representatives.

Sources close to Mr Trump, 76, said he planned to push ahead despite mixed results from his endorsements this year, with losses by celebrity doctor Mehmet Oz in Pennsylvania and Don Bolduc in New Hampshire contributing to Republicans’ failure to win a majority in the US Senate.

Another Trump-picked candidate, former football star Herschel Walker, was forced into a December 6 runoff in his Georgia race against Democratic US Senator Raphael Warnock.

That has raised concerns Mr Trump’s announcement could again hurt Republican chances in a Georgia runoff, similar to the January 2021 runoff that gave Democrats their current majority.

Multiple Trump-aligned candidates who ran on platforms focused on his false claims of widespread election fraud were also defeated.

Conservative columnist Marc Thiessen urged him not to run again.

“That should be a wake-up call for Trump. He cannot win the presidency with his base alone,” Thiessen wrote in a Washington Post column.

“His conduct since losing office has made him unelectable.”

Jason Miller, a Trump adviser, said the former president would urge his supporters to back Mr Walker in a speech he described as “forward looking”.

Mr Trump decided to make his announcement now to provide some excitement for Republicans, Mr Miller said.

A Reuters/Ipsos poll taken before the midterm elections showed 53 per cent of Americans and almost one in four Republicans viewed Mr Trump unfavourably.

The poll showed a similar number of Americans viewed President Joe Biden unfavourably.

Mr Trump planned to launch his campaign nearly two years before the November 5, 2024, election despite these concerns, two sources said.

Mr Pence released a book on Tuesday detailing Mr Trump’s unsuccessful pressure campaign to overturn his 2020 defeat.

Mr DeSantis, who Mr Trump has given the derisive nickname “Ron DeSanctimonious,” handily won re-election last week.

Mr Trump will seek his party’s nomination even as he faces trouble on several fronts, including a criminal investigation into the removal of classified documents from the White House and a congressional subpoena related to his role in the January 6, 2021, US Capitol attack by his supporters.

Mr Trump has called the various investigations politically motivated and denies wrongdoing.

Mr Trump is seeking to become only the second US president in history to serve non-consecutive terms, after Grover Cleveland, whose second stint ended in 1897.

Mr Biden, 79, has said he intends to run for re-election to a second four-year term in office, though he has yet to make a final decision.

-with AAP