Donald Trump has announced his 2020 election slogan will be "Keep America Great!"

The US president, speaking at a a rally in the industrial heartland of Pennsylvania, said: "We can't say 'Make America Great Again' because I already did that. I can't use that in three years, two-and-half years, it's close.

"I'm almost positive, you can never be 100 per cent sure, but our new slogan when we start running is going to be 'Keep America Great...exclamation mark."

Mr Trump added: "I look forward to 2020 because I want to see how far left the person we're running against is going to be."

In a speech that was peripatetic even by Mr Trump's standards he also railed against the European Union on tariffs, which he called "my baby," said he hoped for the "greatest deal" with North Korea, and strongly advocated the death penalty for drug dealers.

President Donald Trump reacts to the crowd while speaking at a campaign rally in Moon Township in Pennsylvania Credit: AP

The president, speaking to thousands of raucous supporters in an airport hangar near Pittsburgh, also said he would love to run against Oprah Winfrey in 2020 because he "knows her weakness".

Amid allegations of an affair with a porn star, which he denies, Mr Trump also lavished praise on his wife Melania, calling her an "amazing first lady" and adding: "You think her life is so easy folks? Not so easy."

A man wears a Trump 2020 campaign button as U.S. President Donald Trump speaks in Pennsylvania Credit: Reuters

At one point he accused the media of portraying him as "some kind of Neanderthal," before countering: "I'm really smart."

Mr Trump was campaigning for Rick Saccone, the Republican candidate for a Congressional by-election in an area of Pennsylvania known as "steel country".

Donald Trump speaks against a backdrop of posters declaring: "Promises Made, Promises Kept" Credit: AP

Next Tuesday's election has become a huge focus of attention as Mr Saccone is losing in a district that voted for Mr Trump by 20 percentage points in 2016.

A Republican loss could signal a bloodbath in the November mid-term elections. Mr Trump's recent announcement of 25 per cent tariffs on steel, and 10 per cent on aluminum, was released at least in part to shore up support for Mr Saccone among current and former steel workers.

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Mr Trump gave an impassioned defence of his tariffs and was drowned out by cheers as he vilified the EU.

He said: "Now, all these countries are ringing up...a lot of you are originally from EU countries, that sounds nice, but the EU they kill us on trade."

President Donald Trump leaves after addressing a rally touting his steel tariffs Credit: Getty

Addressing the EU directly he said: "Open up the barriers and get rid of your tariffs, and if you don't do that we're going to tax Mercedes Benz, we're going to tax BMWs. Cars are the big money item."

Mr Trump was then drowned out by supporters as he appeared to mention other EU products to tax.

He went on to accuse countries of banding together to "screw the US" on trade, adding: "With the EU we're like $100 billion down on trade because we had stupid politicians.

"EU sounds so innocent...it's not innocent. They are very tough and very smart. They have artificial barriers. Not just tariffs, environment and other things like you wouldn't believe. You can't get your product in."

Mr Trump added: "We're saving the steel and a lot of steel mills are now opening up. Not all my friends on Wall Street love it, but we love it. Many plants have announced over the last few days they're opening. Steel is back! And aluminum is back!"

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He said the steel other countries had been dumping in America was "crap," adding: "Your steel is coming back, it's all coming back. The great American comeback continues full speed ahead."

Mr Trump spoke near two giant signs saying "Promises Made" and "Promises Kept". He added: "It's not easy in Washington DC, you've got a lot of evil there but we're getting it out."

The audience was an enthusiastic one, bursting into chants of "USA, USA", loudly and repeatedly jeering the assembled media, and raising the roof at any mention of steel tariffs.

Veering wildly off script Mr Trump mocked the idea of being "presidential" saying it was "easy and boring".

He said: "If I came like a stiff you guys wouldn’t come here tonight."

Donald Trump has unleashed a Hobbesian trade war

Referring to "sleepy eyes" Chuck Todd, a prominent US TV journalist, he added: "He's a sleeping son of a b--c, I'll tell you."

He also called Maxine Waters, a Democrat congresswoman who has called for his impeachment, a "very low IQ individual".

In a lengthy description of how his decision to meet with Kim Jong-un came about, Mr Trump lambasted previous presidents, adding: "Who else could do it honestly (but me)?" He also gave credit to President Xi Jinping of China.

Mr Trump said: "They (North Korea) are not sending missiles up, I believe that they want to make peace. It's time. And I think we've shown great strength. I think that's also important.

"I may leave (the meeting) fast, or we make the greatest deal for the world."

US President Donald Trump supporters attend the Make America Great Again Rally Credit: AFP

Mr Trump was enthusiastic about the death penalty for drug dealers, saying he had asked leaders of both China and Singapore about how they dealt with the problem.

He said: "Probably you'll have some people say that's not nice, but these people are killing our kids and we have to do something. We can't just keep setting up committees."

Turning to potential 2020 rivals he said: "I'd love Oprah to run, I'd love to beat Oprah. I know her weakness, I know her weakness. That would be a painful experience for her."

Donald Trump supporters attends the Make America Great Again Rally Credit: AFP

He then launched into an extended attack on Mr Saconne's Democrat opponent Conor Lamb, a 33-year-old US Marine, saying he was "weak" on crime, immigration, and gun rights.

Mr Lamb has outspokenly supported Mr Trump's steel tariffs.

Mr Trump said: "For getting your vote he's backing tariffs which is my baby. I appreciate his nice words about me, this is Trump country so he says nice things about me. Smart. But don't be conned by this guy Lamb."

After calling him "Lamb the sham" Mr Trump added: "I hear he's nice looking. I think I'm better looking than him though.”