Why Kamala Harris would be Biden’s likeliest replacement

So far, Biden has shown no interest in exiting the race following a halting debate performance that raised questions about his age and abilities. But while some Democrats ponder a variety of governors or lawmakers that could step in as nominee, the likeliest alternative to the current ticket remains one led by Harris.

While that path carries risk because Harris’s poll numbers are as lackluster as Biden’s, she is popular enough among Democrats—particularly among women and Black voters—that casting her aside could cause resentment and division within the party.

“This lady is ready to be president. She really earned her stripes," said Keith Williams, chairman of the Michigan Democratic Party Black Caucus. “I would be angry if you try to leapfrog over a vice president."

Since the debate, Harris has emerged as Biden’s strongest public defender. Immediately after Biden’s poor performance against former President Donald Trump, she did several interviews trying to tone down the panic quickly spreading among Democrats. She then embarked on a cleanup tour on behalf of Biden, telling voters, donors and some celebrities to look past the debate and judge the president on his record.

The election wouldn’t be decided by “one night in June," she told voters in Las Vegas, giving a lengthy defense of the president. “In a real leader, character matters more than style," she said. “Trump does not have the character to be the president."

Harris also countered Trump’s debate responses on abortion rights and the Jan. 6, 2021, riots in a clearer way than Biden did at the debate, while vouching for the president’s abilities. “In the Oval Office, negotiating bipartisan deals, I see him in the situation room keeping our country safe," she said.

Biden and his team have made clear he plans to stay in the race. “President Biden is our nominee, Vice President Harris is our running mate, and we will win this November," said campaign spokesman Kevin Munoz.

Top campaign officials briefed donors in a call Monday evening. A person familiar with the call said the campaign stressed that Biden knows he needs to do better after the debate and that they plan to have him out more. They pledged to stay the course, arguing that voters still care about issues like abortion and democracy and don’t want a return to Trump. Asked how they would respond if polls dip, the person said there wasn’t much of an answer beyond noting that polls fluctuate. Several donors on the call said they weren’t reassured.

Harris and her allies, meanwhile, are being careful to show their loyalty to the president, according to people close to Harris. The vice president will maintain her travel schedule and make the case for Biden, addressing any concerns about the debate, at every stop.

The Democratic National Committee intends to formally nominate Biden as their party’s presidential nominee with the help of a virtual roll call weeks before their Chicago convention in August, a move that would allow the president to appear on the ballot in Ohio.

If Biden were to exit from the race before the nomination, Harris would have some advantages in the field of Democrats who are seen as contenders—which includes names such as California Gov. Gavin Newsom, Michigan Gov. Gretchen Whitmer, Pennsylvania Gov. Josh Shapiro, Maryland Gov. Wes Moore and Sens. Raphael Warnock, of Georgia, and Amy Klobuchar, of Minnesota.

As Biden’s running mate, Harris would be able to use the money already raised by Biden’s campaign committee. The vice president would also have some key endorsements, including from influential Rep. Jim Clyburn (D., S.C.).

“I’m a Biden-Harris person, OK?" Clyburn said after Thursday’s debate. “So I’m not getting away from that. I’m going to be for Biden, if Harris ain’t there, and I’ll be for Harris, if Biden ain’t there."

Donna Brazile, political strategist and former interim DNC chair, also said that if there were an effort to bypass Harris, “there would be an uproar."

“Joe Biden placed her on the ticket for a reason and they won in 2020," she said. “Any attempt to try to change a horse after the voters have selected the team they want to see on the team, there would be total chaos."

Not many pollsters have released surveys testing Harris’s strength against Trump. But Harris garners similar support to Biden’s as a Democratic nominee in the surveys that are available. A New York Times/Siena College poll in February found Harris trailing Trump by 6 percentage points, 42% to 48%, compared with a 4-point deficit when Biden was matched against Trump.

In a Fox News survey last November, Trump led Harris by 5 points, in line with his 4-point lead over Biden.

Similarly, Harris and Biden are viewed about equally in Wall Street Journal polling when asked if voters see them in a positive or negative light. Some 36% viewed Harris favorably in the Journal’s February poll, with 56% viewing her unfavorably—a 20-point gap that was similar to the 22-point gap in views of Biden. Republicans almost universally held a negative view of Harris, as did 61% of independents and 14% of Democrats.

Liano Sharon, a DNC member from Lansing, Mich., who identifies as a progressive Democrat, said he would like to see Biden step aside for an open process at the convention. “I think if Biden throws his support behind her that will make for a very divisive convention," he said of Harris. “It’s ridiculous we’re having this conversation about how difficult this is to do. This is what a party is for."

If the president stays on the ticket, Harris will be under even more pressure to prove to voters she is ready to serve if needed, given the growing concerns about Biden’s age in the aftermath of the debate.

Trump’s campaign has already featured Harris in a TV ad emphasizing the president’s age. “You know who’s waiting behind him, right?" the narrator asks in the ad, with a video of Harris on the screen.

The vice president said in an interview earlier this year: “I am ready to serve. There’s no question about that," Harris responded bluntly. Everyone who sees her on the job, Harris said, “walks away fully aware of my capacity to lead."

Harris’s aides say the vice president doesn’t need to do anything differently to show voters she’s ready for the job.

But in recent months, she has taken more of a public role in the administration, including a greater involvement in adding to her national security portfolio. Harris has joined Biden on calls with Israeli Prime Minister Benjamin Netanyahu, given major speeches on the conflict, and pushed administration officials to articulate more empathy for Palestinians and to focus on a postconflict Gaza plan.

Write to Catherine Lucey at catherine.lucey@wsj.com and Tarini Parti at tarini.parti@wsj.com