WASHINGTON: US President Joe Biden will on Wednesday frame his exit from the 2024 election as a defense of democracy in what could be his last speech to the nation from the Oval Office.

The 81-year-old is to say that the best way forward to unite a divided country is to “pass the torch” to new leadership, according to excerpts released by the White House.

Biden has endorsed Vice President Kamala Harris, 59, to lead the Democratic Party into November's election, after he bowed to weeks of pressure following a disastrous debate performance against Donald Trump.

But in a sign that the election will remain deeply polarizing, Republican Trump lashed out at Harris during a rally as a “radical left lunatic” and described her as a “new victim to defeat.”

In his first speech since announcing his withdrawal, Biden will speak from the Oval Office of the White House, a powerfully symbolic setting used by US presidents for moments of grave historical import.

“The defense of democracy is more important than any title,“ Biden is to say in the primetime speech due at 8:00 pm (0000 GMT Thursday).

“I have decided the best way forward is to pass the torch to a new generation. That is the best way to unite our nation.”

The veteran Democrat was also set to reject accusations that he will be a lame duck for the remaining six months of his one-term presidency.

“Over the next six months I will be focused on doing my job as president,“ he said, adding that he would keep trying to lower costs for families and defend freedoms including the right to abortion.

Biden's withdrawal has upended Trump's campaign, which had previously focused on the president's age and mental acuity. Now it is Trump, 78, who is the oldest candidate in US history.

Trump hit back at Harris in a harsh broadside at a rally in North Carolina, his first since she secured enough delegates to become the de facto Democratic nominee on Monday.

“She is a radical left lunatic who will destroy our country,“ he said, describing her as the “ultra liberal driving force behind every single Biden catastrophe.”

Trump also claimed that Harris wanted “abortions in the eighth and ninth month of pregnancy” as well as “right up until birth and even after birth, the execution of a baby.”

Saying Democratic Party bosses were behind Biden's decision to step down, he accused Harris of covering up Biden's “mental unfitness.”

Earlier Wednesday, the White House denied it was hiding any possible decline in Biden's health prior to his decision to drop out of the election race.

“It is not a cover up. I know that is the narrative that you all want. It is not,“ Press Secretary Karine Jean-Pierre told reporters, adding that Biden's decision has “nothing to do with his health.”

Biden returned to the White House on Tuesday after spending nearly a week with Covid at his Delaware beach home, a period of isolation in which he took the decision to step back.

Republicans have called for Biden to step down altogether, saying that if he is not fit to stand for reelection then he is not fit to serve as president.

Jean-Pierre dismissed that as “ridiculous” and maintained that Biden is no “lame duck.”

The veteran Democrat insists he still has much to offer, such as targeting a peace deal in the Middle East -- a legacy-defining dream that many US presidents before Biden have chased.

Biden, who has been increasingly critical over Israel's military campaign in Gaza, meets Israeli Prime Minister Benjamin Netanyahu at the White House on Thursday.

But in a sign of how quickly things are changing, Netanyahu will sit down separately that day with Harris.

Trump, meanwhile, said in a post on Truth Social that he will meet the Israeli leader Friday at the Republican's Mar-a-Lago resort in Florida.