Trump claims Biden 'abandoned Scranton' when family moved 50 years ago

The troller in chief was at it again in Joe Biden's hometown hours before he will accept the Democratic nomination for president

Mocking the designated Democratic presidential nominee in his hometown, Donald Trump told a rally crowd that Joe Biden "abandoned Scranton" then "sold out" America during a half century in Washington.

"Joe Biden is no friend of Pennsylvania," the president said on a sun-drenched stage in the key swing state, where he trails the former vice president in nearly every poll by a statistically significant margin.

The president attacked Mr Biden during a campaign rally that was also a move to troll the soon-to-be Democratic nominee hours before the former VP will close the Democratic National Convention with a speech during which he will, after decades of trying to secure it, accept the party's presidential nomination.

He predicted Mr Biden will in his big speech "remind us he was born in Scranton."

"Biden left like 70 years ago. He left a long time ago; he as 8, 9, 10. He left 68 years ago," Mr Trump said. "He abandoned Scranton. His parents had something to do with that."

The president then mocked his general election foe as he tries to again win the state and its 20 Electoral College votes in a sates he won by under 1 percentage point in 2016.

"I was born in Scranton, I lived in Scranton," he said in a mocking voice. "Then he left for another state. This Scranton stuff, that's why I thought I'd come here and explain it."

The crowd laughed and applauded.

After former President Bill Clinton on Tuesday night described him as a television-obsessed and ineffective leader, the president admitted to spending time this week watching the Democratic National Convention.

"I watched President Obama last night," he said. "He spied on my campaign and he got caught. ... We're dealing with crazy people on the other side.

RealClearPolitics' average of several polls gives Mr Biden a 5.7 per cent lead in the Keystone State as he prepares to accept the nomination.