Addressing a crowd of thousands on Tuesday at Orlando's Amway Center, Trump complained he had been "under assault from the very first day" of his presidency by a "fake news media" and "illegal witch hunt" that had tried to keep him and his supporters down.

And he painted a disturbing picture of what life would look like if he loses in 2020, accusing his critics of "un-American conduct" and telling the crowd that Democrats "want to destroy you and they want to destroy our country as we know it."

"A vote for any Democrat in 2020 is a vote for the rise of radical socialism and the destruction of the American dream," he said, ripping "radical" and "unhinged" Democrats even as he made only passing mention of any of the men and women running to replace him.

And he appeared eager for a rerun of 2016, spending considerably more time focused on former Democratic rival Hillary Clinton, whose name elicited "Lock her up!" chants, than on his current 2020 challengers, even though she is not on the ballot.

Close by, hundreds of anti-Trump protesters clapped and took photos when a six-metre blimp of a snarling Trump baby in a nappy was inflated.

Some members of the far-right hate group Proud Boys were also spotted marching in Orlando outside the rally.

Trump aides scheduled the launch near the four-year anniversary of the day when the bombastic reality television star and New York tabloid fixture launched his long-shot campaign for president with a famous escalator ride in front of a crowd that included paid actors.

Trump spoke fondly of his 2016 run, calling it "a defining moment in American history".

And he said that, in the years since, he had fundamentally upended Washington, staring down "a corrupt and broken political establishment" and restoring a government "of, for and by the people".

Of course, Trump never really stopped running. He officially filed for re-election on January 20, 2017, the day of his inauguration, and held his first 2020 rally in February, 2017, in nearby Melbourne, Florida. He has continued holding his signature "Make America Great Again" rallies in the months since.

Still, he gave lip service to the hype, telling the Orlando crowd he was standing "before you to officially launch my campaign for a second time."

He asked the crowd whether he should stick with "Make America Great Again" or upgrade. His new campaign slogan - "Keep America Great" - was greeted with boisterous cheers.

Trump is hoping to replicate the dynamics that allowed him to capture the Republican Party and then the presidency in 2016 as an insurgent intent on disrupting the status quo.

Early Democratic front-runner Joe Biden said on Tuesday that Trump's politics are "all about dividing us" in ways that are "dangerous - truly, truly dangerous".

Australian Associated Press