It was an increasingly familiar position for Newsom, who has emerged as perhaps the Biden re-election campaign's most visible representative.
Newsom's role as Biden's cheerleader-in-chief has also boosted his own national profile ahead of 2028, when he is widely expected to consider a White House run.
Whether Biden wins in 2024 or not, Democrats will need a new standard-bearer in 2028, with many in the party calling for a younger, more energetic leader. Newsom is near the top of a list that includes Vice President Kamala Harris, Michigan Governor Gretchen Whitmer and Pennsylvania Governor Josh Shapiro, among others.
Newsom has remained in the spotlight since the debate. On Thursday, he signed a bill raising the minimum wage for the state's half-million fast food workers, a major victory for labor leaders. The news on Friday of U.S. Senator Dianne Feinstein's death meant Newsom will have the job of naming a replacement to help protect Democrats' slim Senate majority.
While the governor has repeatedly denied any interest in usurping Biden as the party's presumptive nominee next year, he would be among the leading alternatives alongside Harris - a fellow Californian - if the 80-year-old Biden were forced to step aside.
"This is a win-win," said Steven Maviglio, a veteran Democratic strategist in California. "It benefits Biden, and it benefits Newsom."
Newsom's office and his political action committee did not respond to requests for comment.
Newsom made headlines last year when he urged Democrats to take a more combative tack, saying the party was getting "destroyed" on messaging by Republicans.
He has taken his own advice to heart. In November, Newsom and Florida Governor Ron DeSantis, who is seeking the Republican presidential nomination, will meet in an unusual one-on-one televised debate, with Fox News host Sean Hannity moderating.
'DRY RUN'
Newsom, elected twice as governor of the country's most populous state, has spent much of 2023 building a national network of supporters. In March, he launched a political action committee, Campaign for Democracy, with $10 million targeting conservative states such as Alabama and Arkansas.
In recent weeks, Newsom has signed or promised to sign a flurry of bills passed by California's Democratic-majority legislature, including a first-in-the-nation excise tax on gun and ammunition sales and a requirement that large corporations disclose their carbon footprint.
But he has also vetoed a few bills backed by progressives, including one that would have barred driverless heavy-duty trucks and another that would have required judges to consider parental acceptance of children's gender identities in deciding custody cases.
Jane Kim, the California director of the liberal Working Families Party, said Newsom deserves credit for many of his achievements. However, she said progressives wish he had done more to push liberal priorities such as single-payer healthcare, a policy he ran on in 2018.
"We've seen him thread that needle, because it's clear he has an eye to a more national profile, not just California voters," she said.
Newsom's decision on Feinstein's replacement could be politically fraught.
He has previously said he would elevate a Black woman if the post became vacant. But he has also said he would not pick one of the candidates running for the seat in 2024, disappointing supporters of U.S. Representative Barbara Lee, who is vying with several other Democrats.
While Newsom has touted California's success in advancing labor unions, climate protections, gun safety and social justice, he would face some significant obstacles in running for national office.
Republicans often portray California as a bastion of leftist extremism. Critics point to persistent homelessness and a lack of affordable housing in some of the state's biggest cities.
DeSantis referred to his November debate with Newsom at an event in Long Beach, California, on Friday, calling the state "the Petri dish for leftism in this country."
Newsom also is a white man in a party whose voters are eager to elevate women and candidates of color, especially after Biden's tenure, said Eric Schickler, a political science professor at the University of California in Berkeley.
"Often when politicians go from the state level to the national level, they really struggle with communicating - you can see that with DeSantis," Schickler said. "To some extent, this is a dry run for Newsom."
