Republican presidential candidate Donald Trump said on Sunday he thinks Vice President Kamala Harris will be easier to defeat in November's election than Democratic President Joe Biden, who earlier in the day stepped aside as his party's candidate.

CNN said the Republican former president made the comments to the network shortly after Biden announced his decision. 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.

The Trump campaign had already begun discussions about how it would redeploy campaign resources for the possibility of Biden's dropping out, a source with direct knowledge of the matter said on Sunday.

Given that any alternative Democratic candidate would likely have different strengths and weaknesses than Biden, that person said, the president's dropping out would require rethinking where to spend ad dollars and where to deploy resources more generally.

Publicly, Trump campaign advisers and allies have been telling reporters they are not worried about facing Harris because they can simply tie her to Biden's record in office, particularly on immigration and inflation. They say they will try to portray Harris, and any of the other candidates being suggested as alternatives for the Democrats, as being to the left of Biden on various policies.

In a statement after Biden dropped out, the Trump campaign said Harris was Biden's "enabler in chief." The campaign said Biden and Harris owned each other's records and "there is no distance between the two."

The official Republican National Committee YouTube channel published a two minute video on Sunday afternoon attacking Harris over immigration policies, alleging she neglected that issue.

In recent weeks, Trump's campaign and some of his allies have launched pre-emptive political attacks on Harris to try to discredit her amid talk she could replace Biden atop the party's 2024 presidential ticket.

In March 2021 Biden said Harris would lead efforts with Mexico and Central American nations to address illegal immigration.

Republicans have seized on that to accuse her of failing to stem the flow of millions of migrants crossing illegally into the United States, although she was never directly responsible for securing the southern border.