The former vice president’s team denounced the President after he echoed North Korea’s Kim Jong-un, who called Mr Biden a “low IQ idiot”.
Speaking to reporters in Japan on Monday, Mr Trump said: “Kim Jong-un made a statement that Joe Biden is a low IQ individual. I think I agree with him.”
Mr Biden waited until Mr Trump was back in America before responding, sticking to a tradition dating back to the Cold War that US presidents should not be criticised while abroad.
A statement issued last night by Mr Biden’s deputy campaign manager Kate Bedingfield said: “The President’s comments are beneath the dignity of the office.
To be on foreign soil, on Memorial Day, and to side with a murderous dictator against a fellow American and former vice president speaks for itself.”
Mr Trump tweeted today: “I was actually sticking up for Sleepy Joe Biden while on foreign soil. Kim Jong-Un called him a ‘low IQ idiot’, whereas I related the quote as a much softer ‘low IQ individual’. Who could possibly be upset with that?”
A new poll gives Mr Biden a lead of more than 20 points over Bernie Sanders, his nearest rival for the Democrat presidential nomination.
This is despite Mr Biden being accused of inappropriate exchanges with women. One state legislator complained he came up behind her and kissed the back of her head.
At an event in Houston, Texas, yesterday, he raised eyebrows when he told a schoolgirl: “I’ll bet you’re as bright as you are good-looking.” Last month, Mr Biden said he would be more mindful about his contact with others.
He has focused on lambasting Mr Trump rather than taking on his Democrat competition.
At the launch of his campaign in Philadelphia, he criticised Mr Trump for embracing leaders such as Russia’s Vladimir Putin and Mr Kim, saying: “Tyrants all over the world are using Trump’s language to stifle dissent.”
CNN says Mr Trump has snubbed advice from aides to ignore Mr Biden.