Washington DC - Donald Trump had harsh words for President Joe Biden on Sunday after he announced that he was dropping out of the election and endorsing Vice President Kamala Harris as the new Democratic nominee.

Joe Biden was "not fit to run" and "is certainly not fit to serve" as president, his Republican rival Donald Trump said after the president said he was dropping out of the White House race.

"Crooked Joe Biden was not fit to run for President, and is certainly not fit to serve – And never was! ... We will suffer greatly because of his presidency, but we will remedy the damage he has done very quickly. MAKE AMERICA GREAT AGAIN!" Trump posted on his Truth Social network shortly after Biden's announcement.

Donald Trump's campaign team further lashed into Vice President Kamala Harris after Joe Biden endorsed her to take his place atop the Democratic Party's presidential ticket, saying she would be "even worse" than the outgoing leader.

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"Harris will be even worse for the people of our nation than Joe Biden. Harris has been the Enabler in Chief for Crooked Joe this entire time. They own each other's records, and there is no distance between the two," the campaign said in a statement.

Later, it was the turn of Trump's Vice Presidential pick JD Vance to heap scorn on Biden and Harris.

"Joe Biden has been the worst President in my lifetime and Kamala Harris has been right there with him every step of the way," the Republican senator wrote on X, saying Harris had "lied for nearly four years about Biden's mental capacity."

"President Trump and I are ready to save America, whoever's at the top of the Democrat ticket. Bring it on," Vance concluded.

Former President Bill Clinton and ex-Secretary of State Hillary Clinton praised Joe Biden's decision to abandon his White House reelection bid and threw their support behind Vice President Kamala Harris to take up the baton.

Lauding Biden's "extraordinary career of service," the Clintons said in a joint statement that they were "honored" to join him in endorsing Harris as the Democratic nominee "and will do whatever we can to support her."

"Nothing has made us more worried for our country than the threat posed by a second Trump term. He has promised to be a dictator on day one," they added.

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Former President Barack Obama also praised Biden's decision to drop out of the White House race on Sunday, saying it was a testament to the leader's "love of country" – but warned of "uncharted waters" ahead of the November 5 election.

At the other end of the political spectrum, the top Republican in Congress said that Joe Biden must resign as president "immediately" after the president's announcement.

"If Joe Biden is not fit to run for president, he is not fit to serve as president. He must resign the office immediately. November 5 cannot arrive soon enough," said House Speaker Mike Johnson in a statement.