"Harris will be easier to beat than Joe Biden would have been," Trump told CNN.
Trump and his campaign later also attacked Biden and Harris on social media while saying Biden was unfit to continue serving as president.
Biden ended his reelection campaign on Sunday after fellow Democrats lost faith in his mental acuity and ability to beat Trump. Biden endorsed Harris to replace him as the party's candidate.
Biden had faced growing doubts about his reelection chances after a weak and faltering performance in a televised debate against Trump late last month.
On his Truth Social platform on Sunday, Trump said Biden "was not fit to run for President, and is certainly not fit to serve."
Other top Republicans, including House of Representatives Speaker Mike Johnson, also said Biden was not fit to serve as president and finish his term if he was stepping aside as the Democratic presidential candidate. Johnson explicitly called on Biden to resign.
Trump, in a post on his Truth Social platform, said: "We will suffer greatly because of his (Biden's) presidency, but we will remedy the damage he has done very quickly."
Trump and Biden had been mostly tied in polls, but after the debate some polls showed Trump narrowly ahead of the president in a match-up for the November elections.
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