US president Donald Trump has taken a fresh swipe at one of his leading Democratic rivals in the 2020 election, telling a rally in Pennsylvania that native son Joe Biden had abandoned them by representing Delaware in the senate.

In fact, Mr Biden moved to neighbouring Delaware with his family when he was a boy, and later represented the state in the senate for more than three decades. He maintained ties to Pennsylvania over the years.

Mr Trump’s Pennsylvania visit, intended to boost Republican congressional candidate Fred Keller over Democrat Marc Friedenberg in a special election for an open House seat, had as much to do with helping his own re-election prospects as it did with pushing Mr Keller over the line.

“We’ve got to win tomorrow, Fred,” Mr Trump told a cheering crowd at a hangar in Williamsport Regional Airport.

The president’s visit to the key battleground state came two days after former US vice president Mr Biden held a campaign rally in Philadelphia.

Mr Trump told the crowd: “He left you for another state, and he didn’t take care of you.”

He also referred to the former vice president by the nickname he has coined for him: Sleepy Joe.

“Sleepy Joe said that he’s running to, quote, ‘save the world,'” Mr Trump said. “Well, he was. He was going to save every country but ours.”

Mr Biden said in Nashville, Tennessee, that he is running on a pledge to restore the soul of America.

He has frequently spoken on the campaign trail about the president’s divisive rhetoric and said another four years of Mr Trump would “fundamentally change the character of this nation”.

The president uses his campaign rallies to disparage various Democratic rivals for the presidency, but he has been heavily focused on Mr Biden, suggesting he may be worried about the possibility of facing off next year against the long-serving politician.

The president, who spoke in the open air with Air Force One behind him, highlighted the American economy’s performance under his leadership and suggested those numbers make him virtually unbeatable.

“Politics is a crazy world, but when you have the best employment numbers in history, when you have the best unemployment numbers in history … I don’t know, how the hell do you lose this election, right?” Mr Trump said.

The current unemployment rate of 3.6% is actually the lowest since 1969, when it stood at 3.5%. Unemployment was even lower than that in the early 1950s, and much lower – under 2% – during three years of the Second World War.

Mr Keller himself offered a rousing endorsement of Mr Trump, saying he wants to go to US congress to be a vote for the president. Mr Keller told the president that the people of Pennsylvania “have been behind you since Day One, and, Mr President, our support for you is as strong today as it ever was”.

He added: “In 2016, Pennsylvania put Donald Trump over the top. And in 2020, we’re going to do it again.”

Mr Biden is making a big play for his native Pennsylvania, opening his presidential bid in Harrisburg and capping a three-week campaign rollout with Saturday’s event in Philadelphia, the city where he also established his campaign headquarters.

In the fight to deny Mr Trump re-election, no places will matter more than Pennsylvania, Michigan and Wisconsin, three states the Republican president carried by razor-thin margins in 2016.

Mr Trump campaigned in Michigan and Wisconsin earlier this year.