Calls are mounting for US President Joe Biden to step aside as the Democratic Party candidate, following his poor performance in a presidential debate against Republican candidate Donald Trump.

The 81-year-old president’s faltering presentation during the June 27 debate has sparked questions and doubts about his ability to lead for another four years. During the debate, Biden appeared to lose his train of thought and at times looked exhausted or confused.

A damage control campaign has since followed, with the president and his backers scrambling to assure voters that he is fit and won’t be stepping aside.

But some within the Democratic camp seem dissatisfied; the list of representatives calling for a Biden drop-out is now growing.

Here’s what we know about how the debate has affected Biden and who a replacement candidate might be:

Why did the debate affect Biden?

The most significant impact of Biden’s debate performance is that it intensified doubts about his capacity – which now threatens his ability to win the presidential elections in November, experts note. That adds to simmering dissatisfaction with how Biden’s government has handled inflation, the Gaza war and immigration on the southern border with Mexico.

An Ipsos poll showed that after the presidential debate, voter confidence in Biden’s mental fitness dropped from 28 percent to 20 percent.

Presidential debates have been an important part of US elections since first airing in 1960 and have proved crucial in shifting voters towards the better speaker.

After the Sunday debate, voter preferences have shifted towards Trump by an average of 3.5 points, according to one analysis by the Brookings Institution, a US think-tank. Prior to the debate, Trump was ahead of Biden by an average of 1.5 points.

Another opinion poll by Ipsos showed that post-debate voter support for both candidates was tied at 40 percent.

Analysts say even small shifts are crucial in what’s set to be a tight election in a country deeply polarised along political ideologies.

Which Democrats have called for Biden to be replaced?

Within the Democratic Party, five members of the US House of Representatives have called for Biden to step aside: Representatives Seth Moulton (Massachusetts), Lloyd Doggett (Texas), Raul Grijalva (Arizona), Mike Quigley (Illinois) and Angie Craig (Minnesota).

On Sunday, a separate group of Democrats at a virtual meeting convened by the House minority leader, Representative Hakeem Jeffries (New York), discussed how to pressure Biden to step aside, according to The Associated Press.

What happens if Biden drops out of the race?

It’s nearly unprecedented for a nominee to step down so close to the elections. If that happens, Democrats would have to field another candidate.

President Biden could step down and simply nominate Vice President Kamala Harris to carry on, but her approval ratings have not been much higher than Biden’s, casting doubt on her ability to floor Trump.

However, Harris would inherit Biden’s campaign funds, whereas any other nominees would be starting their campaigns from scratch, campaign finance lawyer Steve Roberts told The Hill.

If Biden steps down before the August 19-22 Democratic National Convention in Chicago, one of the more likely scenarios is that the party will nominate another candidate to run in his place. Democrats last used this method in 1968, after the leading candidate, Senator Robert F Kennedy, was assassinated just weeks before the convention.

The Democratic Party could also force Biden out by invoking the 25th Amendment, which provides that if the president dies, is disabled or resigns, the vice president assumes the role. The VP and a majority of Democrats in Congress could invoke the act by declaring to the speaker of the house and the Senate that the president is not fit, and should be removed. The president could contest this declaration. If the VP and a majority of lawmakers disagree, the matter would be put to a two-thirds majority vote in Congress.

Could donors force Biden to quit?

Donors have considerable power, but it’s unclear how they might affect a potential nomination.

Some donors to the Biden campaign have joined the list of people asking Biden to step down, including former PayPal CEO William “Bill” Harris.

Harris, who donated $620,000 to the Biden campaign in 2020, told reporters last week that Biden’s departure is “inevitable”. Harris is now leading a group of donors pledging $2m to fund a series of debates among Democratic candidates that would set the stage for another candidate to be selected.

Some are threatening to withhold donations to the Democrats until Biden steps down, putting pressure not just on the president, but the entire party.

In an opinion piece on the website Deadline, Hollywood producer Damon Lindelof urged fellow donors to impose a “DEMbargo”.

Lindelof wrote: “When they text you asking for cash, text back that you’re not giving them a penny and you won’t change your mind until there’s change at the top of the ticket.”

For now, there have been no such calls from the Biden campaign’s biggest donors. Political action group Future Forward USA Action and venture capital firms Greylock Partners and Sequoia Capital are the top three donors for the Biden campaign, according to the website OpenSecrets.

Who could replace Biden and how would they fare against Trump?

Some have put forward Michelle Obama as a likely nominee, but the former first lady has repeatedly said “no”. Yet 50 percent of those polled said they would vote for Obama if she were on the ticket, putting her seven points ahead of Trump.

More likely candidates, however, include:

• Kamala Harris: VP Harris, 59, is a straightforward replacement for Biden and likely a top runner. A former prosecutor and California senator, Harris could appeal to young voters, as well as supporters from Black and other minority communities (she is of Indian and Jamaican descent). A vocal advocate of abortion rights, she could also pull more female voters. Representative Adam Schiff told reporters he felt Harris would win “overwhelmingly” if she ran. However, Harris’s approval ratings are not much higher than Biden’s or Trump’s. In the Ipsos poll, 43 percent of voters said they’d vote for Harris. Another poll by 538 puts Harris’s approval ratings at 37 percent.
• Gavin Newsom: Governor Newsom of California has been a longtime campaign surrogate for Biden-Harris and has recently been campaigning for the duo in the swing states of Michigan and Pennsylvania. Newsom, who had previously voiced ambitions to run in the 2028 elections, is now being floated as a potential replacement for Biden. Some have also pointed to a potential Harris-Newsom ticket in November. Ipsos polled him at 39 percent.
• Gretchen Whitmer: Governor Whitmer of the battleground state of Michigan is a star of the Democratic Party. The two-time governor shot to fame in 2020 after clashing with then-President Trump over her strict COVID-19 lockdown policies. Her popularity in Michigan was cemented after she won 52 percent of the vote in the 2022 elections. Whitmer was originally floated to be the party’s nominee in the 2028 elections, but the Ipsos poll showed that 36 percent of US voters would elect her if she appeared on a ticket. It is not yet clear which Democratic representatives will back Whitmer.
• JB Pritzker: Pritzker has been governor of Illinois since 2019, and has been pointed out as a potential replacement for Biden, largely because of his ability to take on Trump. He garnered attention after his ferocious attacks on Trump during the former president’s criminal trial in New York. Following Trump’s conviction in May, Pritzker called him a “felon” and “an old man with an orange spray tan who fell asleep at his trial”. Pritzker is from one of the US’s wealthiest families and holds progressive policies around access to abortion, minimum wage and expanding healthcare access. Some 34 percent of voters say they’d choose Pritzker, according to Ipsos.
• Josh Shapiro: Pennsylvania Governor Shapiro previously served as the state’s attorney general. He comfortably won his election in the mid-Atlantic swing state in 2022. Since taking office, he has had positive approval ratings and is seen as a level-headed leader – an advantage for the Democrats over Trump in the battleground state. One issue that could split voters is Israel’s war on Gaza. Shapiro, who is Jewish, has passionately spoken in support of Israel, adding to a deep divide in the Democratic Party. Shapiro saw a 57 percent approval rating in a May poll by Siena College and local media.