MOON TOWNSHIP, Pennsylvania (AFP, NYTIMES) - It's barely 2018, but US President Donald Trump is gearing up for a 2020 re-election campaign, with a familiar sounding new slogan: "Keep America Great!"

Addressing a rally in a Pittsburgh suburb, Trump endorsed a Republican candidate in a closely-watched upcoming special congressional election, before turning to his own political future.

"Our new slogan when we start running - can you believe it, in two years from now - is going to be: 'Keep America great, exclamation point,'" he said.

Trump's snappy but controversial "Make America Great Again" slogan dominated the 2016 election atmosphere, and his supporters - many of whom wore red hats bearing the phrase at the rally - have used it as a catchphrase for the president's first year in office.

Trump did mention the Republican running in Tuesday's special election, Rick Saccone, a handful of times. A good man, he called him.

But for almost all of his 75-minute performance in front of a raucous crowd packed into a hangar, it was in-his-element Trump, vintage 2016: rambling and fiery, boastful and jocular - the part of being president that he loves perhaps the most.

Ricocheting off the teleprompter, the president showed the kind of free-flowing attitude that his aides have said they expect to see throughout this year.

He ticked off what he said were his achievements - some coming in recent days - on a laundry list of issues like North Korea, trade and the economy, and attacked his predecessors for their failures on the same.

He spoke admiringly of foreign laws imposing the death penalty on drug dealers and seemed to brush aside the notion of due process as he spoke of US officers grabbing gang members "by the neck" and throwing them in the paddy wagon.

He derided past presidents as stiffs and lousy entertainers. He pummelled his favourite targets: Democrats and the "fake" news media. And, always the showman, he dropped a bit of news, revealing the slogan for his 2020 re-election campaign: Keep America Great. With an exclamation mark, he said.

"Is there anything more fun than a Trump rally?" he asked on the heels of a whirlwind week in which his chief economics adviser said he was resigning, he formalised steep tariffs on steel and aluminium that angered business leaders and allies, and abruptly announced, through South Korean officials, that he intended to meet with Kim Jong Un, the North Korean leader.

But while the rally seemed to reveal a president happier than he had been in weeks, his mentioning of the new slogan, which he had alluded to for months but never before highlighted, pointed to the challenge he will face in 2020.

While he will argue that he has done much to restore the country to greatness, he will have to make a case that there is enough left undone that he should be given another term to finish the job.

He said he would welcome a battle against another television celebrity who has been a fantasy candidate for Democrats: Oprah Winfrey. But if she runs, he warned a bit ominously, "I know her weakness."

"Wouldn't we love to run against Oprah?" he asked. "I would love it. I would love it. That would be a painful experience for her."

The rally, which had been rescheduled following the deadly Parkland, Florida, shooting last month, capped a frenzied stretch of fundraising and campaigning on behalf of Saccone, who is locked in a tight race with the Democratic nominee, Conor Lamb.

"He's an extraordinary person," Trump said of Saccone, dismissing Lamb as "Lamb the Sham."

"The people of Pittsburgh cannot be conned by this guy Lamb, because he's not going to vote for us."

It was an opening for Trump not only to try to apply his political sway in the year's first special election - one being looked at as a bellwether for the November midterms - but also to seize the opportunity to promote his new tariffs before a steel-country base of supporters far more receptive than the officials and pundits in Washington.

"Steel is back," he proclaimed more than once. "Aluminium is back."

He also, after receiving some boos upon first mentioning Kim, the North Korean dictator, said he thought the North Koreans "want peace".

He said his unprecedented meeting could go two very different ways: "We may leave fast" or strike one of history's greatest deals.

While the rally was not specifically advertised as one for Saccone, a loss by him would mark the third time that Trump had thrown his political might behind a candidate without success.

With his low poll numbers, Republican leaders and the White House are trying to determine how to handle his desire to campaign in the midterms. But he was a hit with the crowd on Saturday.

He retreated to fan favourites: reminiscing about his Electoral College victory, and saying that there is a "lot of evil in Washington" and that he is "getting it out".

To cheers and laughter, Trump hurled some old and new slurs toward his opponents as he complained about his media coverage and perceived Democratic obstruction.

Trump also reprised his criticism of liberal Senator Elizabeth Warren, whom he mocks as "Pocahantas", and Senator Bernie Sanders, the far left independent who challenged Hillary Clinton for the Democratic nomination in 2016.

"I look forward to 2020 because I want to see how far left the person is going to be that we're going to run against," Trump said.

Trump boasted of his appeal to female voters, falsely claiming that he had received 52 per cent of women's votes in 2016. (He received 52 per cent of votes from white women.)

"Hey, didn't we surprise them with women during the election?" he said, noting that his opponent was a woman.

"Remember? 'Women won't like Donald Trump.' I said, 'Have I really had that kind of a problem?'"

During the campaign, a number of women levelled allegations of sexual misconduct against the president. And as president, he continues to be shadowed by accusations of an affair by the pornographic film actress known as Stormy Daniels.

Still, he tossed compliments toward his wife, Melania Trump. "You think her life is so easy, folks? Not so easy," he said. And the president himself?

Surrounded by his supporters, his foils in the news media, and a campaign soundtrack, he was at ease.