President Donald Trump arrives to speak at his re-election kickoff rally at the Amway Center in Orlando. Picture: Evan Vucci/AP

Washington - US President Donald Trump launched his 2020 re-election campaign on Tuesday at a mass rally in Florida, blasting his opponents as radicals, presenting himself as a political outsider and slamming the media.

The president, however, did not announce any concrete new policies, focusing instead on stoking fears that his rivals would destroy the US way of life, alleging they support mass migration to the country.

Trump repeatedly slammed illegal immigration, which he claimed was responsible for a decline in the middle class, along with past international trade agreements.

"We are going to keep making America great again and then we will indeed keep America great," Trump said, as he began to switch his campaign slogan, with the older one still visible on many thousands of red hats and shirts in the audience.

At one point Trump, a former reality TV star, had the crowd of some 20,000 people vote, through howls and cheers, on whether to keep the 2016 campaign slogan "Make America Great Again" or change to the newer "Keep America Great," with supporters approving the latter.

The president's repeated "fake news media" blasts drew strong applause from the audience, who chanted against broadcaster CNN as Trump pointed to television cameras.

Polls are showing the president may face an uphill battle to retain control over the White House next year.

Trump entered a crowded 2016 presidential race as an outsider vowing to "drain the swamp," and was seen as a long-shot, with polls regularly predicting his defeat.

He now has the backing of his Republican Party establishment and enjoys widespread support among party members, but even after serving two years in the White House, Trump in Florida continued to slam "Washington insiders" and the "rigged" political system.

His broadsides against socialism and doubling down on conservative stances on nationalism, abortion, family, the judiciary and guns were positively received by the loyalist crowd.

"Our radical Democrat opponents are driven by hatred, prejudice and rage. They want to destroy you and they want to destroy our country as we know it. Not acceptable, not going to happen," Trump said in one of his toughest attacks.

The president has pledged to begin to deport millions of migrants in the country illegally starting next week, but has yet to reveal more concrete plans, despite teasing ahead of the rally that he would use the pulpit in Florida for a big announcement.

A large campaign operation is already in place. While Trump has kept on some of the people who helped him get elected two years ago, there has also been a high turnover rate within his team, with several figures facing a range of legal issues.

The election is still 17 months off. The president faces little internal competition but the rival Democrats have more than 20 presidential contenders vying for the nomination, with early signs of bickering and feuds within the party's ranks.

Florida is a crucial swing state that Trump narrowly won in 2016. No president has been elected without Florida, the third most populous state, in nearly 100 years.

Trump also has deep ties to the state, where his Mar-a-Lago resort functions as a second home. He has held numerous rallies in Florida since becoming president and stepped up aid for the state. His native New York voted against him two years ago.

The Orlando rally comes as polls show the president behind the main Democratic presidential contenders nationally and in Florida.

A Quinnipiac poll, released Tuesday, showed Trump trailing the early Democratic front-runner, former vice president Joe Biden, by nine percentage points in Florida.

Senator Bernie Sanders and Senator Elizabeth Warren, the more progressive candidates, are also ahead of Trump in Florida.

Nationally, Trump is behind Biden by 10 points, according to a Fox News poll released Sunday, despite relatively good economic numbers, such as a low unemployment rate, though wages have remained stubbornly low.

dpa