US Vice President Kamala Harris lauded President Joe Biden for his service to the country in her first public appearance on Monday since he abruptly abandoned his re-election bid and endorsed her as his successor.

"Joe Biden's legacy over the last three years is unmatched in modern history," Harris said at a White House event to honour college athletes. "Every day, our president, Joe Biden, fights for the American people and we are deeply, deeply grateful for his service to our nation."

Harris did not specifically refer to her new status as the leading Democratic candidate for president, after Biden stepped aside on Sunday under growing pressure from fellow Democrats.

Campaign officials and allies have already made hundreds of calls on Harris' behalf, urging delegates to next month's Democratic Party convention to join in nominating her for president in the November 5 election against Republican Donald Trump.

Biden's departure was the latest shock to a White House race that included the near-assassination of former President Trump by a gunman during a campaign stop and the nomination of Trump's fellow hardliner, US Senator J.D. Vance, as his running mate.

Harris said in a post on X that she would visit what had been the Biden campaign headquarters in Delaware – now the Harris campaign headquarters – on Monday afternoon. She said Biden, who tested positive for COVID-19 last week, was feeling better.

Virtually all of the prominent Democrats who had been seen as potential challengers to Harris have lined up behind her, including Governors Gretchen Whitmer of Michigan, Gavin Newsom of California and Andy Beshear of Kentucky. (Reuters)