With US President Joe Biden dropping out from the forthcoming presidential election on Sunday and endorsing Vice President Kamala Harris as the Democratic Party's new nominee, Donald Trump wants the money "reimbursed" he spent on his campaign so far.

The announcement wasn't completely unexpected, especially after pressure mounted on Biden following his poor performance in the first presidential debate last month.

Reacting to the same, Trump Sunday said the incumbent president is "certainly not fit to serve."

He asked if the Republican Party should be "reimbursed for fraud" for the resources and money it spent campaigning against President Joe Biden.

"So, we are forced to spend time and money on fighting Crooked Joe Biden, he polls badly after having a terrible debate, and quits the race. Now we have to start all over again," the 45th President of America wrote on Truth Social.

"Shouldn't the Republican Party be reimbursed for fraud in that everybody around Joe, including his doctors and the Fake News Media, knew he was not capable of running for, or being, President? Just askin'?" he added.

In a letter posted on X, Biden said it had been the "greatest honor of my life" to be president. He said he would address the nation later this week. "While it has been my intention to seek reelection, I believe it is in the best interest of my party and the country for me to stand down and focus solely on fulfilling my duties as President for the remainder of my term," he wrote.

Later, he offered his "full support and endorsement" for Harris, with his campaign filing official notice to change its name to "Harris for President."