Donald Trump questioned Joe Biden's faith in God as he launched a deeply personal attack on his presidential rival.
Even for a US leader known for his blunt criticism, Mr Trump's remarks stood out and they signalled how contentious the campaign may get over the coming months.
"He's against God. He's against guns. He's against energy, our kind of energy. I don't think he's going to do too well in Ohio," he said on a visit to the state.
Former vice president Mr Biden's campaign spokesman responded, saying his faith is "at the core of who he is".
The president was expected to promote the economic prosperity that much of the nation enjoyed before the coronavirus pandemic, and to make the case that he is best suited to rebuild a crippled economy.
But Mr Trump's handling of the outbreak has weakened his bid for a second term, causing him to spend time and resources in a state he won easily in 2016 but now could be in danger of slipping away.
Covid-19 already altered the trip even before President Trump landed, with word that Republican governor Mike DeWine had tested positive for coronavirus.
Mr DeWine had planned to meet with Mr Trump and join him on a visit to the Whirlpool factory in north-west Ohio. Mr DeWine's office said the 73-year-old governor had no symptoms and was returning to Columbus.
Mr Biden's campaign spokesman Andrew Bates responded to President Trump's comments by saying: "Joe Biden's faith is at the core of who he is; he's lived it with dignity his entire life, and it's been a source of strength and comfort in times of extreme hardship."
Mr Bates also accused President Trump of using a Bible "for his own cynical optics as he sought to tear our nation apart at a moment of crisis and pain", a reference to when federal law enforcement officers drove protesters out of Lafayette Square shortly before a photo opportunity in which the president held a Bible.
