Concerned Democrats at Biden rally consider alternatives.

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

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

However, with some major Democratic donors and lawmakers urging Mr. Biden to exit the presidential race, even his staunchest supporters in Madison are open to the idea of a possible replacement and what that might entail.

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

She emphasized her admiration for Mr. Biden’s 17-minute speech, which she described as 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 intense scrutiny following a disastrous debate performance last week, marked by a hoarse voice and several moments of losing his train of thought.

At 81, the president is facing growing doubts about his mental acuity and ability to defeat Trump, 78, in November's election.

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

Yet, the raucous crowd, which waited through several opening speakers and an hour-long delay, was tinged with a sense of unease.

“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, referencing the Democratic Party's signature color.

Mr. Leffler suggested that selecting a new candidate could have unexpected benefits. “If you go through some sort of open process, you can re-energize 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 parties' respective presumptive candidates.

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

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 targeted 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 a significant motivator for many who attended 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 select someone new, he said.

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

“I think Biden can win,” he said.