The campaign has not disclosed the reason for the poll or their intended use of the results. However, it is speculated that the team may be gathering information to present to Biden, indicating whether he remains the strongest standard-bearer for his party.

This development comes amid increasing pressure on Biden to step aside, with a growing number of prominent lawmakers calling for his withdrawal or suggesting he reconsider his candidacy. While some of Biden's senior aides believe Harris is unelectable, supporters of the vice president argue that she may connect better with voters.

Jennifer O'Malley Dillon, Biden's campaign manager, and Julie Chavez Rodriguez, his chief of staff, wrote in a memo to the president's campaign staff about the "path forward": "The path ahead is clear. There is no indication that anyone else in the field would do better against Trump. Hypothetical polls of substitute candidates are unreliable. The only Democrat who we know for certain is running and has the ability to defeat Trump is Joe Biden."

Biden's political future may hinge on his performance during a press conference on Thursday at the NATO summit in Washington. This conference will be Biden's longest appearance since his shaky debate performance two weeks ago, and lawmakers, officials, and supporters have indicated they will be watching closely.

Prior to the press conference, Biden is sending some of his senior aides to Capitol Hill to smooth things over with anxious Democratic senators. The president's team expressed optimism earlier in the week after addressing Democratic critics through a public letter to Congress, a cable TV call, and a phone call with one of his supporters.

Former House Speaker Nancy Pelosi, Chuck Schumer, and Rep. Hakeem Jeffries, the House Democratic leader, are three of the most influential figures in the discussion surrounding Biden's potential withdrawal.

The question of what would happen if Biden steps down has caused unease within the Democratic Party, with many members doubtful of Harris' ability to unite a broad coalition to defeat Trump in November.

Since the debate, Biden's inner circle has been limited to his family and a very small group of his closest aides, effectively insulating the president, and it is unclear how aware he is of his declining standing among Democrats.

Meanwhile, Harris has been careful to display her full loyalty to Biden, even as her supporters have floated the idea of her replacing Biden in the race, with some even suggesting potential running mates for her.