Asked by a reporter if Biden, 81, commits to serving his full second four-year term if he defeats former President Donald Trump in the November polls, spokeswoman Karine Jean-Pierre responded: "Yes."
Biden has been facing calls to withdraw his candidacy since his poor performance against Trump in their first presidential debate on June 27.
If reelected, the Democratic president will be 86 years old when he leaves office.
A day earlier, Jean-Pierre denied reports that Biden is being treated for Parkinson's disease after visitor logs showed that a leading neurologist who specializes in the malady visited the executive mansion eight times in as many months.
On Tuesday, she said the meeting between Biden and the neurologist in January was not related to medical care.
Asked about congressional Democrats who said he should drop out of the race, she said there are those who back him.
"We respect members of Congress. We respect their view. But I also want to say there is also a long list of congressional members who have been very clear in their support of the president," she added.
Earlier, House and Senate Democrats held separate meetings to discuss the viability of Biden's candidacy, with some Senate Democrats expressing deep concern about the presidential race, CNN reported.
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