For a few minutes on Monday afternoon, Gov. Gavin Newsom of California was just like any other politician with national ambitions, making his way through a small crowd at a rest stop in New Hampshire. He greeted local officials. He touched a baby. He was handed a cinnamon sugar doughnut.
“Everyone’s hyping these up,” he said, taking a large bite. “Actual real life, no B.S., not a politician talking. That’s next level.”
A television microphone intruded. “Governor, should the president step aside?”
Mr. Newsom chewed for a moment, reiterated the next-levelness of the doughnut, then turned to the microphone. “The answer is no.”
It was back to work for President Biden’s hardest-working surrogate. The half-eaten pastry was handed off to a local Biden campaign staff member. The doughnut would have to wait.
The stop in New Hampshire capped a days-long tour of battleground states on behalf of Mr. Biden, a trip Mr. Newsom described on Monday in almost evangelical terms: “I’ve got to get out there every day — every day — over and over and over, delivering the message of his accomplishments, but also a compelling future.”
In the 12 days since Mr. Biden’s debate performance threw his re-election bid into disarray, fissures of doubt have emerged in the party he rose to command four years ago. Democratic leaders, even as they call into cable shows and issue public statements of support for the embattled president, seem to be eyeing the exit.
Outside Washington, Mr. Newsom is perhaps Mr. Biden’s most effusively loyal ally. He is certainly the only one who spent an extended Fourth of July weekend in the competitive states of Michigan, Pennsylvania and New Hampshire.
“I decided instead of just rolling over and giving up, that I would step up,” he said on Monday, speaking to reporters corralled into an ice-cream parlor by Biden campaign staff members. “I took my holiday weekend — it wasn’t easy to be away from the kids. I said there was something more important.”
Mr. Newsom, who is widely seen as holding presidential aspirations, has little to lose by putting himself out there for Mr. Biden, as the wealthy, entrenched leader of a powerful and deeply blue state.
He also has a taste for political combat. Long outspoken in his belief that Democrats are not doing enough to aggressively counter Republicans, he has debated Gov. Ron DeSantis of Florida and sparred with Fox News hosts over the past year, straying far from his state to warn voters about the risks of a second Trump administration.
“Gavin Newsom has said he has subzero interest in running for president right now,” said Paul Mitchell, a longtime friend of the governor’s who runs a political data firm in California. “His role has been as a surrogate. He’s a voice for Democrats nationally. This is exactly what he should be doing.”
Going on the road to support the sitting president is not a risky proposition, Mr. Mitchell said. “I just don’t think there’s a downside.”
When Mr. Biden gathered Democratic governors at the White House last week to discuss the path forward for his campaign, Mr. Newsom was the only leader of a state west of Minnesota to attend in person.
After Mr. Biden began the meeting by saying he intended to stay in the presidential race, the first question went to Gov. J.B. Pritzker of Illinois, who asked Mr. Biden how he planned to proceed, according to governors who attended and others who were briefed on the session.
Mr. Newsom went next and offered enthusiastic support for Mr. Biden, telling the president that he was the party’s best candidate — an echo of the Biden campaign’s own messaging, the governors and others briefed said.
That meeting was practically a job requirement. But New Hampshire, as they say, is a place nobody in politics goes by accident, and Mr. Newsom’s mission of defending the president’s record aligns neatly with his own ambitions.
“Governor Newsom is getting ready for his next stage in life,” said Lou D’Allesandro, a longtime Democratic state senator in New Hampshire. “I’m sure he’s looking at the prize.”
Mr. D’Allesandro said the California governor was also prominent enough to help assuage voters about Mr. Biden. For the Biden campaign, he said, “it’s the right move.”
Long before the debate, Mr. Newsom had been scheduled to speak Monday night at a fund-raiser for Democrats in the New Hampshire Legislature. But in typical candidate fashion, mingling at the rest stop outside Manchester kept him late, as he spoke with supporters and posed for pictures, inexplicably clutching an open cup of hot coffee in the sweltering heat.
Tim Platt, 65, a school administrator from Concord, thanked him for his defense of Mr. Biden. Mr. Newsom said, “He’s had our back for decades, and that’s how I was raised, honestly.”
A few moments later, a man shouted across the food court: “Go back to California, you carpetbagger!”
Reid J. Epstein, Shawn Hubler and Maggie Haberman contributed reporting.