As the curtain closed on the worst performance of Joe Biden’s career, journalists at the debate in Atlanta were sprinting to one figure in the backstage spin room.

It was not the US president, but the man many Democrats are now talking up as his potential replacement: Gavin Newsom, California’s governor.

Mr Newsom, long feted as a potential future president, is now being given serious attention by party insiders alarmed by Mr Biden’s appearance on stage on Thursday night.

The California governor was in Atlanta, Georgia, to act as a campaign surrogate for Mr Biden in his first 2024 showdown with Donald Trump.

But after a disastrous night for the US president, during which he froze mid-sentence and struggled to deliver cogent responses, Mr Newsom was being viewed as more of a potential stand-in than support act.

Dozens of reporters swarmed around him moments after the debate, bypassing Trump campaign surrogates altogether. Many asked if he would put himself forward for president. 

“Everyone is talking about you as a possible nominee now,” one said – with Mr Newsom smiling as he brushed the question off.

At 56, Mr Newsom is a quarter century younger than Mr Biden and an adept communicator.

He has the credentials to run for the Oval Office too. He runs the most populous state in the country – and the world’s fifth largest economy.

Pressed on whether Mr Biden should step down, Mr Newsom made no bones about the president’s poor performance.

Yet he said the chatter about replacing the party’s leader was “unhelpful” and “unnecessary”. He told MSNBC: “You don’t turn your back because of one performance. What kind of party does that?”

Those who know Mr Newsom best are frank about his ambition to one day occupy the Oval Office.

The son of a state appeals court judge, he set up his own wine business before being elected mayor of his native San Francisco.

He was briefly married to Kimberly Guilfoyle, a former TV commentator who is now engaged to Donald Trump’s eldest son, Donald Jnr.

Mr Newsom’s current wife, Jennifer, is an actor and documentary filmmaker, with whom he has four children.

He was re-elected to his second and final term as governor of California in a landslide in 2022.

With no political threats to fend off in the Golden State, he has focused on increasing his national recognition, giving speeches in other states, and running national TV adverts.

He has amplified his political profile by wading into this year’s White House race, casting himself as an attack dog who is prepared to aggressively take on prominent Republicans.

His strategy has included challenging Ron DeSantis, Florida’s governor and one-time GOP 2024 presidential candidate, to a debate.

Mr Newsom has also signed a raft of liberal legislation, particularly around abortion, that could help him win over Democratic voters in a contested presidential primary.

The actions caused some unease in the White House, where aides feared he could launch a challenge against Mr Biden, although he publicly and privately assured the president otherwise.

He has become a high-profile defender of the president. Yet his ability to acquit himself on the debate stage and on cable news, in a way Mr Biden simply cannot, has painted an uncomfortable contrast for the Biden campaign and inadvertently underscored the president’s vulnerabilities.

Mr Newsom has tacked to the Left of the Democrat party, which may dampen appeal among centrist voters who backed Mr Biden in 2020.

He has enacted more than a dozen laws aimed at making California a sanctuary for women seeking abortions in states where the procedure is now banned.

Mr Newsom has also championed green policies, ordering a ban on the sale of new fossil-fuel cars by 2035.

And he has looked to ease obstacles for illegal immigrants, making California the first state to offer health insurance for all undocumented immigrants.

Along with Mr Newsom, several other prominent Democratic governors are being touted as potential replacements.

They include Gretchen Whitmer, who secured a convincing victory in the swing state of Michigan.

The 52-year-old has been a persuasive campaigner on abortion access and other policy priorities for the party.

Mr Newsom’s fellow Californian, Kamala Harris, the vice president, would of course be on any short list of replacements.

Members of Mr Biden’s cabinet, including Pete Buttigieg, the transportation secretary and Gina Raimondo, the commerce secretary, are also considered viable contenders.

Nevertheless, Democrats who are intimately acquainted with Mr Biden insist there is little that can be done to dissuade him from seeking a second term.

Insiders suggest the two voices the president plays closest attention to – his wife Jill and his sister Valerie – remain staunchly supportive of his re-election bid.

David Axelrod, a former chief strategist for Barack Obama, told CNN: “The idea he would walk away from this is pretty remote. But his job got harder tonight.”