Providence, Rhode Island - Sheldon Whitehouse, a senator from Joe Biden's Democratic Party, has asked for reassurances about the president's health, saying the American public wanted to be sure his dismal debate performance was just an "anomaly."

In an interview aired Monday on WPRI, a local CBS affiliate in Whitehouse's home state of Rhode Island, the senator said: "Like a lot of people I was pretty horrified" by Biden's debate performance against Republican rival Donald Trump.

In the Thursday night debate, Biden sometimes lost his train of thought, gave halting responses, or stared blankly with his mouth agape.

"People want to make sure that this is a campaign that is ready to go on and win, that the president and his team are being candid about his condition, that this was a real anomaly and not just the way he is these days," Whitehouse said.

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On Friday, the 81-year-old president defiantly sought to reassure supporters.

"I would not be running again if I didn't believe with all my heart and soul I can do this job because, quite frankly, the stakes are too high," he said in a fiery speech in North Carolina.

For months, the White House has downplayed any concerns about the president's physical and mental health, despite the fact that his gait has visibly slowed and stiffened, and his sentences sometimes trail off.

While Democratic Party heavyweights are still publicly backing Biden, nerves are jangled among both his supporters and his donors, some of whom have criticized Biden's aides for being opaque about his condition.

Others have asked Biden to respond directly to the concerns, by either taking part in press conferences or sitting down for interviews with journalists.

On Monday, Biden spoke briefly at the White House about a landmark Supreme Court ruling on presidential immunity. After he finished his scripted remarks, a journalist asked Biden if he planned to withdraw from the presidential race.

He left the room without answering.