Yes, she conceded to a few dozen donors in San Francisco, the president's performance was "not his finest hour." However, she said, "the outcome of this election cannot be determined by one day in June."

It's not the kind of rhetorical two-step that any running mate would want to be practicing just over four months before the election, but Harris has little choice. She's trying to help Biden stave off a death spiral in their campaign for a second term after the 81-year-old president's faltering debate appearance magnified concerns about his ability to beat Donald Trump and serve another four years.

The moment could hardly be more delicate for the 59-year-old Harris, who is the first woman, Black person and person of South Asian descent to serve as vice president. Although some Democrats have pointed to her as a logical successor if Biden steps aside, others are drawing up wish lists of potential replacements that don't include her on the ticket at all.

Depending on how the coming days and weeks unfold, her work on Biden's behalf could lead to a political dead end, or secure her future within the party by providing fresh opportunities to prove her doubters wrong.

Chad Griffin, a member of the campaign's national finance committee, said the White House was fortunate to have a vice president that is "tough as nails" and "out there defending the president and talking about the stakes of this election."

"They're a team," he said. "And we're increasingly seeing the other half of that team."

Biden and Harris had one of their occasional private lunches on Wednesday, and afterward they held a call with campaign staff to vow that they would push forward together.

"Joe Biden has devoted his life to fighting for the people of our country," Harris said, according to a campaign official who was on the call. "In this moment I know all of us are ready to fight for him."

Risks and rewards as Harris elevated

None of this comes without political danger for Democrats. Harris' favorability ratings are low, and she's a frequent target for Republicans, who say she's waiting in the wings in case Biden isn't able to complete a second term. "Vote Joe Biden today, get Kamala Harris tomorrow," said an advertisement from Trump's campaign.

But for now, Harris is Biden's top advocate as he faces calls to shake up his campaign or end his reelection effort. Her presence has been a balm for some anxious Democrats, such as the donors who gathered for a fundraiser in Los Angeles on Saturday.

Harris paced the stage with a handheld mic -- Biden sometimes uses teleprompters even during fundraisers -- as she acknowledged "the elephant in the room," meaning Thursday's debate. Then she swung her focus to Trump, who she described as a liar and a threat to democracy.

"You see how much hasn't changed?" Harris said. "So, let's deal with what we've got, right?"

Griffin said "there was this sigh of relief" from donors afterward. Some said "we want to see more of that" or "the gloves are really off."

Only 39% of U.S. adults have a favorable view of Harris, which is in line with Biden's 40%, according to an AP-NORC poll conducted in June. However, her unfavorable rating is 49%, lower than Biden's 57%, and 12% said they weren't familiar enough with Harris to have an opinion of her.

The numbers mean there's risk and reward in emphasizing Harris, with room for public opinion to swing in either direction.

Next in line for the presidency

There's no easy way for Democrats to force Biden to relinquish the nomination, which is scheduled to be finalized with a virtual vote before the party's convention in Chicago in August. There's also no precedent for a major party presidential candidate to drop out of the race at this point.

But if Biden steps aside, there's a wide array of Democrats who are primed for a shot at the White House, including California Gov. Gavin Newsom, Illinois Gov. J.B. Pritzker and Michigan Gov. Gretchen Whitmer.

However, there are clear advantages for Harris. Under campaign finance rules, she could easily tap the money that's been raised for Biden's reelection. She also has hard-to-obtain foreign policy experience, after crisscrossing the globe on the White House's behalf and meeting with dozens of world leaders.

Rep. Jim Clyburn, an influential Democrat from the key primary state of South Carolina and a co-chair of Biden's campaign, said he would back Harris if the president wasn't in the race.

"This party should not, in any way, do anything to work around Ms. Harris," he told MSNBC.

The vice presidency can be a trap for ambitious politicians, and they're more likely to ascend to the presidency because of death than get elected on their own.

Oftentimes they're too closely tied to the current administration to represent a path forward, or voters are accustomed to seeing them as a sidekick rather than a leader, said Joel K. Goldstein, a vice presidential historian.

"The vice presidency is the best springboard," he said. "But being vice president doesn't mean that you're going to automatically get the nomination, or going to get the nomination without an effort."

During an interview with The Associated Press last year, Harris dismissed the idea of her replacing Biden as "hypothetical," saying "Joe Biden is going to be fine, so that is not going to come to fruition."

However, she said every vice president has needed to be ready to step in if necessary.

"I'm no different," Harris said.

Harris' task

Harris and Biden advisers said the vice president's role hasn't shifted since the debate. She was already leading outreach to key demographic groups, such as Black people and young voters, and pushing for reproductive rights after the Supreme Court ended the nationwide right to abortion.

Harris has also helped Biden fend off questions about his age before, such as when special counsel Robert Hur released his report on the president's handling of classified documents. Although Hur concluded that no criminal charges were warranted, he described Biden as a "well-meaning, elderly man with a poor memory."

The following day, Harris called Hur's report "gratuitous, inaccurate and inappropriate."

But there's no question that the spotlight on Harris has brightened since the debate. A preview came on Thursday shortly after the debate ended.

Her team had previously arranged for a series of television interviews, which is standard practice for big moments like the State of the Union. But as Biden faltered, it quickly became apparent that Harris would be the White House's first line of defense.

She helped set the tone for the campaign's response, saying that Biden had "a slow start" and shifting the focus to his overall record.

"I'm not gonna spend all night with you talking about the last 90 minutes when I've been watching the last 3.5 years of performance," she said.

The interviews drew some over-the-top praise from commentators who compared Harris' coherence with Biden's convoluted answers.

The vice president "has been right under their nose," said Jamal Simmons, Harris' former communications director. "This is who she is."

"People who are on Team Harris are very familiar with her being underestimated," he said.

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Associated Press writer Linley Sanders contributed to this report.