US President Joe Biden leaves the stage after the debate with Donald Trump on June 27. (Source: Getty Images)

Before the debate, Reuters said that in this 90-minute face-to-face confrontation, President Biden will have the opportunity to show Americans who are skeptical about his health that he is still capable of serving as President for another 4-year term.

However, some Democrats who watched the debate expressed "disappointment" with President Biden's performance. He seemed to have difficulty refuting former President Trump and failed to take strong positions on issues such as abortion, which are strong points for Democrats.

NBC News even quoted a Democrat, who was once a staunch supporter of Mr. Biden, as saying: “It’s time to talk about an open convention and a new party candidate.”

“There was a sense of shock about the way he came out at the start of this debate,” said White House official David Axelrod, a former top campaign adviser to former President Barack Obama. “He seemed a little disoriented. … There will be discussions about whether Biden should continue.”

However, many Democrats have defended Mr. Biden's performance in the debate. US Vice President Kamala Harris issued a statement saying that Americans should focus on what the White House boss has done for the country during his time in office, instead of focusing on how he debated on stage.

She expressed displeasure at the criticism directed at President Biden after the debate, but said it was "part of the game" and that American voters would make their choice on November election day, not during the debate.

Jen O'Malley Dillon, Biden's campaign manager, issued a statement praising the President for "presenting a positive, win-win vision for America's future."

California Governor Gavin Newsom also expressed his support for Mr. Biden and affirmed that he "never turned his back" on the President.

"I've had a lot of contact with the president. I know him, I know what he's accomplished over the last three and a half years. I know the president's ability, his vision. I have no concerns about him," Newsom said.

Until now, there has been no precedent for American parties to replace their presidential nominees. Party rules make it nearly impossible to replace nominees without their consent.

In addition, this action will make the results of the votes of voters in the recent primary elections meaningless. Incumbent President Joe Biden won nearly 99% of the total delegates in the Democratic primary elections.

However, the Democratic National Committee's charter does provide some provisions in the event that a party candidate becomes incompetent or decides to resign.

The only viable scenario for Democrats to have a new nominee is for Mr Biden to decide to withdraw, something he has repeatedly said he would not do.

According to NBC News , there is no evidence that Democrats would accept a change in nomination without President Biden’s consent. But even if they did, there is no mechanism to replace a nominee before the convention, and certainly no way to appoint a new one.