US President Donald Trump launched his 2020 re-election campaign with a rally in front of about 20,000 supporters packed into a Florida arena, chanting "USA, USA!"

The crowd in Orlando erupted with cheers as Mr Trump entered.

Even if dismal early poll numbers show he faces a difficult race, Mr Trump goes into the re-election fight buoyed by a strong economy and confidence in the fierce loyalty of his right-wing base.

President Trump told the crowd that together they had formed "a great political movement" that had "stared down a broken and corrupt political establishment".

The Orlando rally was sold out and Mr Trump promised the crowd, forming a sea of red "Make America Great Again" baseball caps, the political version of a rock concert.

"People have never seen anything like it (unless you play a guitar). Going to be wild," he tweeted ahead of the event.

Supporters lined Orlando's downtown footpaths all day, waiting in tents and chairs overnight to be the first in the door.

"This is a historic event, we would not miss this for anything," said one fan, David Meloney.

Florida will be one of the key swing states in 2020 if Mr Trump is to defeat the nominee chosen from a field of 23 Democratic hopefuls.

And ahead of the rally, he pushed several of the core issues in his populist, nationalist platform.

In one tweet, the president complained, as he does almost daily, about the "Fake News" failing to admit to his popularity.

In another, he attacked the European Union for what he says is currency manipulation making it "unfairly easier" for the US ally when it comes to trade.

His biggest focus could well be illegal immigration.

This is a theme he has hammered repeatedly during his presidency and he appears ready to ramp this up further with an announcement on Twitter that he wants the deportation of "millions of illegal aliens".

"They're going to start next week," he told journalists later, responding to reports that his tweet had taken his own immigration authorities by surprise.

The election is set to be polarised, with some on the left pushing hard for President Trump's impeachment 

After more than two drama-filled years at the White House, the fast-talking real estate salesman bets that a roaring economy and his nationalist message on immigration will persuade Republicans and enough centrist blue collar workers to vote him in again.

But there is no question that a lengthy probe into Mr Trump's murky dealings with Russia and his divisive, bruising style have left him wounded.

A wide range of polls show Mr Trump lagging far behind Democratic frontrunner Joe Biden, who is campaigning on a promise to return the country to what he portrays as the calmer, gentler days of Barack Obama, under whom he served as vice president.

A Quinnipiac University poll released yesterday showed Mr Biden leading Mr Trump 50-41% in Florida, while Senator Bernie Sanders is up 48-42% over the president in the state.

Polls so far in advance have limited value and in 2016 they famously failed to predict Mr Trump's defeat of Democrat Hillary Clinton. If anything, the surveys point to another bitterly fought, tight race.

But in a sign of frayed nerves, Mr Trump has lashed out at what he calls "fake" polling, while multiple US media reports say that his campaign has fired several of its own pollsters.

Democrats are also fired up, with the party's most active wing veering left and a vocal minority pushing for Mr Trump's impeachment.

So Mr Trump will need everyone from his fervent base to turn out, setting the stage for a sharply polarised election.

Setting the tone, Mr Trump's ultra-loyal, core Republican supporters in red "Make America Great Again" baseball caps chanted "USA!" while booing journalists assigned to cover the event.

These are the voters who steered Mr Trump to improbable victory in 2016, when he faced the polished and experienced Mrs Clinton.

By homing in on working-class white Americans and tapping into their grievances over globalisation and the liberal elites, Mr Trump successfully ate into the Democrats' own base.