Mr Newsom, 56, faced off against his Republican nemesis Ron DeSantis, 45 on Thursday, in a prime-time TV debate on Fox News moderated by Sean Hannity.
Unlike the Florida governor, Mr Newsom is not running for president next year. But his challenge to Mr DeSantis, along with a string of other high-profile forays into national politics, have fuelled suspicion over what is driving California’s golden governor.
Sources within Mr Newsom’s circle have dismissed claims he is setting himself up as a viable alternative to the 81-year-old Mr Biden, insisting that he is simply offering his skills as a highly effective cheerleader for the president’s re-election bid.
Garry South, who ran Mr Newsom’s first run for governor in 2008, said he and many other Democrats were pleased Mr Newsom had “stepped up”, because the president was not aggressively rebutting Republican “mistruths” and his deputy was not “effective” at fulfilling her role as his “designated attack dog”.
Mr Newsom is “a better messenger [for Democrats] than either Biden or Harris are, to be honest”, he added.
The Democratic strategist, who remains in close contact with the California governor, described him as a formidable debater who would “eat DeSantis for lunch”.
The debate between Mr Newsom and Mr DeSantis, the Florida governor, is a high-stakes venture for both men.
Mr DeSantis is hoping to revive his stalled Republican primary bid before the contest kicks off in Iowa in January, with polls showing him trailing far behind the frontrunner, Donald Trump.
Unlike his main contender for second place, Nikki Haley, Mr DeSantis has concentrated his campaign resources in Iowa, giving the first primary state make-or-break status for his presidential bid.
Meanwhile, Mr Newsom is seeking to prove his mettle in the most high-profile debate of his career. He is widely expected to mount a presidential bid in 2028.
Gil Duran, another California-based operative, said he believed Mr Newsom’s advisers were “really surprised” that Mr DeSantis accepted the California governor’s challenge to debate.
“Because there’s just really no universe in which a presidential candidate should consent to debate someone who’s not running for president. A hard rule in politics is do not punch down,” he said.
“If DeSantis weren’t so desperate for press coverage, this wouldn’t be happening,” he added.
The Democrats’ heir apparent
Despite long-running rumours that Mr Newsom was preparing to challenge Mr Biden in the Democratic primaries, the California governor has chosen instead to become a leading “surrogate”, heading out to defend the president on the airwaves.
He appeared at the second Republican primary debate – hostile territory for a liberal governor – to criticise all the candidates from the “spin room” in southern California’s Simi Valley in September.
A Biden campaign source told The Telegraph that Mr Newsom was one of many surrogates the campaign had deployed, including JB Pritzker, governor of Illinois, and Gretchen Whitmer, governor of Michigan.
But Thursday’s event has ignited a debate in Washington over whether Mr Newsom is taking orders from Biden, or running his own “shadow campaign”. While Mr Newsom’s appearance at Simi Valley was coordinated with the Biden campaign, his debate with Mr DeSantis was his own idea.
“His team reached out and confirmed that we wouldn’t have any issues with it, which we did not,” said the source. “He gave us a heads up.”
Sources close to Mr Biden have poured cold water on the idea that Mr Newsom could launch an outright challenge to the president, citing the difficulty of setting up a new campaign and raising donations in time for the primaries in January.
“The only people who were talking about the governor running against Joe Biden or primary-ing Joe Biden was the beltway media,” said an aide.
Mr Duran, who worked for Mr Newsom’s predecessor in the governor’s mansion, Jerry Brown, has a slightly different perspective. Mr Newsom was serving as lieutenant governor at the time.
“He does have a bit of a history of ankle biting his superiors,” Mr Duran said, suggesting that Mr Newsom’s headline-grabbing antics this year were not initially rooted in altruism.
But he believes Mr Newsom has since “whittled down his aspirations” and to a new role as a surrogate and Biden booster.
“The Biden campaign is now supportive of his activities,” he said. “It sort of got reframed from Newsom being a rogue trying to run his own shadow campaign to Newsom now serving the Biden campaign as a top surrogate.”
Climate change visit to China
Mr Newsom made headlines last month when he travelled to Beijing for talks with the government about climate change policy.
While it is not unusual for state governors, who represent major economies in their own right, to travel internationally to meet foreign leaders, no other governor has done so in China for four years.
Xi Jinping, the Chinese premier, surprised observers by meeting Mr Newsom himself – his only meeting with a US governor since 2017.
The visit appeared to be designed to lay the groundwork for a highly anticipated meeting between Mr Xi and Mr Biden in the governor’s home state earlier this month.
Green energy is a major priority for the White House, and the US president has invested significant political capital in boosting domestic clean power production under his “Bidenomics” agenda.
Echoing the president’s own rhetoric of “derisking, not decoupling” with China, the California governor told reporters: “Divorce is not an option.”
Golden State liberal
By tying himself to the administration, Mr Newsom has managed to expand his profile to a national audience ahead of an expected run in 2028.
Aware that his record in California could come to define his political future, Mr Newsom has taken an aggressive, hands-on approach to tackling the crime and drug epidemic gripping his state. He has also attempted to temper the liberal state legislature, blocking progressive bills on transgender children, caste discrimination and the legalisation of magic mushrooms.
Given his ability to read the national mood, supporters say the telegenic, 6ft3 governor has the talent to go all the way to the White House – if he chooses to.
“He’s a terrific performer – that’s one of his main assets,” said Mr South.
“He has a mind like a steel trap. He remembers every fact that he ever came into contact with – and he can pull them out at will to make his point or to destroy someone else’s point.”
Mr Newsom also has an interesting backstory. The son of a state appeals court judge, he set up his own wine business before entering politics and becoming elected mayor of San Francisco.
A dyslexic, Mr Newsom takes most of his briefings in audio format, and has written a book for children with the condition. His career might have taken a different trajectory: he had a promising future career as a baseball player until an injury in the late 1980s.
His ex-wife, Kimberly Guilfoyle, is now engaged to Donald Trump Jr. His current wife, Jennifer, is an actor and documentary filmmaker who once dated George Clooney. Last year, she took the stand to accuse Harvey Weinstein of rape. A jury was unable to reach a verdict on the charge.
Battle of the deputies
The debate on Thursday between Mr Newsom and Mr DeSantis is likely to be revealing of their differences.
As governors, they have pursued converse policies: while Mr DeSantis has imposed a six-week abortion ban in Florida, California has cemented its status as an abortion safe haven. While the Golden State imposed strict lockdown early in the pandemic, the Sunshine State remained defiantly open.
Thursday’s showdown may offer a glimpse into presidential campaigns of the future.
Mr DeSantis looks unlikely to topple Mr Trump in 2024 – but could be the Republican frontrunner in 2028. Mr Newsom appears content to wait his turn – but won’t stay silent in the meantime.