Former US president Donald Trump went after his successor President Joe Biden on immigration and border security along with other issues Sunday in his first public speech since leaving the White House.

Trump's last major appearance was during an outdoor speech in Washington on Jan 6, the day his supporters stormed the Capitol in Washington to protest the certification of Biden's Nov 3 victory in the Electoral College. The exhortations led to Trump's second impeachment.

Sunday's unrepentant two-hour speech, which started nearly an hour later than scheduled, was delivered to an audience at the annual CPAC (Conservative Political Action Conference) meeting in Orlando, Florida. This year's CPAC probably has drawn the most attention in its history, due to the theatrical nature of Trump's speeches and curiosity about his political future.

"We all knew that the Biden administration was going to be bad — but none of us imagined just how bad they would be, and how far left they would go," Trump said, salting his speech with sarcasm. "Joe Biden has had the most disastrous first month of any president in modern history.

"In just one short month, we have gone from America first to America last," he said, referencing one of his political themes.

"Who knows? I may even decide to beat them for a third time," an emboldened Trump said to wild applause, underscoring his refusal to accept the 2020 election result.

Trump, 74, was twice impeached by a Democratic-led House of Representatives in the past year and acquitted both times in the Senate, the second time by a Democratic-controlled Senate, which fell 10 votes short of the 67 votes needed for a conviction.

"The Republican Party is united. The only division is between a handful of Washington DC establishment political hacks, and everybody else all over the country," Trump said Sunday.

As for talk that Trump would form a third party in a break with establishment Republicans, he called the notion "fake news" on Sunday.

Trump lashed out against the new administration's policies on immigration and security along the US border with Mexico.

"Joe Biden defunded the border wall and stopped all future construction," Trump said. "Catch-and-release has been restored. … You take their name and you release them into our country."

Trump said that during their debates last year, the Democratic president didn't talk about his more than 40 forthcoming executive orders and actions, adding, "maybe we could have found something if the media did their job, and they don't".

Trump also spoke at length about election procedures.

"We need election integrity and election reform," he said.

Trump railed against the Supreme Court, three of whose majority conservative members he appointed, for not hearing his campaign's challenges to changes in election procedures, particularly in the swing state of Pennsylvania.

"They didn't have the guts or the courage to make the right decision," he said of the court, while calling for voter ID, verification of signatures, proof of US citizenship and limits on mail-in ballots.

He also called for the immediate reopening of schools, many of which have been holding classes remotely or with limited in-person instruction.

"Joe Biden sold out America's children to the teachers unions," Trump charged.

"Trump left us in a place where nearly 500000 had died of COVID," Senate Majority Leader Chuck Schumer tweeted Sunday evening. "Trump left us after inspiring, directing, propelling a mob to the Capitol to violently prevent transfer of power. But Republicans in Congress still joined a CPAC where Trump repeated his Big Lie today. Unconscionable."

Trump also vented about Biden's reversals of the former's various executive actions with the new president's own and railed against the halt in construction of the Keystone XL pipeline, the rejoining of the World Health Organization, "cancel culture", censorship on social media, and the participation of transgender athletes in girls' sports.

Earlier Sunday, South Dakota Governor Kristi Noem criticized Dr Anthony Fauci, now Biden's chief medical adviser. Noem, a Republican, said her state focused on hospital capacity rather than coronavirus-case numbers.

"I don't know if you agree with me, but Dr Fauci is wrong a lot," she said to applause.

Fauci, the director of the National Institute of Allergy and Infectious Diseases, called Noem's remarks "unfortunate".

"It is not really helpful," Fauci said Sunday on CBS' Face the Nation. "Because sometimes you think things are going well, and just take a look at the numbers. They don't lie. I'm sure that you can get a standing ovation by saying I'm wrong."

Trump has had a contentious relationship with the Senate's top Republican, Mitch McConnell, who bluntly blamed Trump for the Capitol riot, which later led to the former president calling the veteran legislator from Kentucky a "dour, sullen ... hack".

But last week, McConnell said that if Trump were the 2024 GOP presidential nominee, he would support him.

Senator Mitt Romney of Utah, a frequent Trump critic, also said last week that he believed Trump would win the 2024 Republican nomination in a landslide were he to run.

Trump has sought to politically punish those he has deemed disloyal, including the No. 3 House Republican, Liz Cheney of Wyoming, daughter of former vice-president Dick Cheney, along with others who voted to impeach him.

The CPAC was mostly a tribute to Trump, complete with a golden statue in his likeness. It also wasn't unnoticed that the event, normally held in Washington, was moved to Florida, where Republican Governor Ron DeSantis has touted the state's largely open businesses and schools amid the pandemic.

Trump also is contending with lawsuits and investigations, such as one involving his tax records in New York City, which the Supreme Court granted access to last week.