As President Biden tries to retain the Democratic nomination following his poor debate performance, prominent California figures, particularly Vice President Kamala Harris and Governor Gavin Newsom, are trying to bolster the embattled incumbent in the race.

Newsom will campaign for the president and raise money for local politicians in New Hampshire on Monday, the first state in the nation to hold a presidential primary, while Harris will rally around Asian American voters in Nevada, another key early voting state, on Tuesday.

Both Democrats, members of the same generation who made their careers in the trying arena of San Francisco politics, have publicly and vocally supported Biden, but as a growing number of Democrats call for Biden to drop out of the race, they are among the names most frequently mentioned as potential replacements if the president decides not to seek reelection.

On Sunday, Rep. Adam B. Schiff of Burbank became the latest Democrat to voice concerns about Mr. Biden's candidacy.

“I think his performance on the debate stage has rightly raised questions among the American people about whether the president has what it takes to beat Donald Trump, and this is an existential election,” Senate candidate Schiff said on “Meet the Press.” “It can't be close. The only reason it's close is because of the president's age.”

Biden reiterated on Monday that he has no plans to withdraw from the race and wrote a letter to Democrats urging them to unite behind his candidacy and to defeat Trump.

“The question of how to move forward has been debated for over a week now, and it's time to end it,” Biden wrote. “We have one job: defeat Donald Trump. We are 42 days away from the Democratic National Convention and 119 days away from the general election. Any weakening of resolve or lack of clarity about the agenda ahead will only help Trump and hurt us. Now is the time to come together, move forward as a party, and defeat Donald Trump.”

Growing calls among Democrats for Biden to drop out of the race have sparked speculation about who could be a likely candidate to replace him.

Harris is clearly the front-runner to be Biden's No. 2. She has also run for president before and has been on the national stage for years. But Newsom's name has also been floated. Neither has publicly signaled a desire to replace Biden, but their appearances in early voting states have made the speculation inevitable.

“I think Newsom is thinking about 2028, whereas Kamala is talking about tomorrow or the day after,” said Dante Scala, a political science professor at the University of New Hampshire. “I think Newsom is smart enough to realize that if it's not Biden, it's going to be Harris. If Harris fails, he'll continue playing the long game he's doing. If Harris is the nominee and it doesn't work out, it opens the door for a nominee in 2028.”