The hundreds of die-hard Democrats who turned out to see Joe Biden in Wisconsin on Friday didn’t need much convincing.

The US president received an enthusiastic response to his loudly delivered remarks at the rally in Madison, especially when he attacked his Republican rival Donald Trump.

But as some major Democratic donors and lawmakers call on Mr Biden to exit the presidential race, even some of his most ardent supporters here in Madison are keeping an open mind about whether he might be replaced - and what might come next.

“It’s OK to change our minds,” said Catherine Emmanuelle, 44, who paused and considered her thoughts carefully before outlining her opinion.

She stressed that she was impressed with Mr Biden’s 17-minute speech, which she called a “presidential litmus test”.

“But if something happens in three days or a week or three weeks, we shouldn’t be afraid of having a conversation about change,” she told BBC News.

Mr Biden is under tremendous scrutiny after a disastrous debate performance last week, marked by a hoarse voice and several instances where he lost his train of thought.

The president, 81, is facing a tide of doubts about his mental acuity and ability to beat Trump, 78, in November's election.

Friday’s rally, held in this reliably Democratic town in a critical swing state, was an indication of the support Mr Biden still has in many parts of the country.

But the raucous crowd, which waited through several opening speakers and a hour-long delay from the planned start time, was also shot through with low-grade anxiety.

“I’m worried about his capacity to beat Trump,” said Thomas Leffler, a 33-year-old health researcher.

"As he gets older, I think it’s going to increasingly be an issue. But I’ll vote blue no matter what," he said - a reference to the Democratic Party's signature colour.

Mr Leffler suggested that picking a new candidate might have unexpected benefits.

“If you go through some sort of open process, you can re-energise people, and show that there’s a process better than what Republicans have, which is basically just to bow down to Donald Trump,” he said.

Earlier this year, both the president and Trump secured the delegates needed to be their party's respective presumptive candidates.

The Democrats' nominee will officially be chosen at the Democratic National Convention in Chicago from 19-22 August.

On Friday, Mr Biden was defiant about staying in the race, telling the crowd: "I am running and going to win again.”

Some of the biggest cheers on Friday came when the president directly went after his predecessor.

“Trump is not just a convicted criminal,” he said. “He’s a one man crime wave.”

The prospect of a second Trump administration was an animating factor for many who came to the rally.

“During the debate, he told a bunch of lies,” said Greg Hovel, 67. “How is that any worse than what Biden did?”

Mr Hovel said he believed the country was in a “great place” and that Mr Biden didn’t get enough credit for his economic and pandemic recovery policies.

“At this point, in the next six weeks, the Democratic Party is going to have to make up its mind” whether to retain Mr Biden as their candidate or pick someone new, he said.

But the president’s performance on Friday further bolstered something he strongly believed, even before the speech.

“I think Biden can win,” he said.

With additional reporting by Jonathan Csapo