Kamala Harris is Biden's heir apparent, but some Democrats not sold: ANALYSIS

Vice President Kamala Harris heads into the dog days of summer as the clear favorite to replace President Joe Biden as the Democrats' White House nominee. But there are still members of her party left to convert into believers.

Harris is entering a moment of deep political uncertainty with significant advantages over other would-be replacements.

She is the sitting vice president; as a 59-year-old Black woman with Indian heritage, she could appeal to key demographics in the Democratic coalition; after a rocky start, she has found her voice in office, particularly around abortion; and she has support from Democratic heavyweights like Biden and Rep. James Clyburn, D-S.C.

On the other hand, she is being relentlessly attacked by Republicans over the administration's handling of immigration, which Biden put under her portfolio, and memories of her dysfunctional 2020 campaign still give some Democrats pause over whether she can put together a campaign to face off against former President Donald Trump.

Taken together, Harris likely offers a higher ceiling among Democrats than Biden, after his cataclysmic debate last month. But Democratic anxiety over electability is still high with her at the top of the ticket, especially with Trump coming off a politically advantageous streak that saw him score legal wins, anoint a successor to his movement and literally dodge a bullet.

"What Kamala offers that President Biden didn't is upside. She has potential to win over new voters and has a higher ceiling than President Biden did," one source familiar with the Biden-Harris campaign's thinking said. "I think there's certainly some convincing to do."

Harris, who said Sunday it is her "intention is to earn and win this nomination," enters the race at a time when Democrats are clamoring for someone -- anyone -- who can take on Trump without have to answer constant questions about mental fitness or age. Harris certainly gives them that.

While Republicans are painting her as complicit in what they claim is a cover-up of Biden's declining cognitive abilities, his departure from the race likely puts the storyline on his age on the backburner. And while Democrats' post-debate hullabaloo sent Biden into a tailspin, it offered Harris a chance to showcase her media skills, including the night of the debate, when she deftly parried worries over the president's performance.

"I think anybody who is wondering what kind of messenger Vice President Harris can be under duress, they should just watch the Anderson Cooper interview from the night of the debate. She was really crisp on message. She was really strong, really forceful. I mean, she looked and sounded like a candidate who could deliver a winning message," said Democratic strategist Peter Giangreco.

"We will immediately have a better messenger because Vice President Harris can talk about the next four years and not be stuck talking about the last three weeks. So, it's an upgrade."

On top of that, Harris has grown in her role.

Democrats of all stripes concede that she had a rough start to her tenure as vice president, seemingly unsure of what her political role was as Biden's No. 2. But she ultimately found her groove, especially as a vocal defender of abortion rights. And while her poll numbers aren't significantly higher than Biden's, supporters told ABC News there's more room to grow than with the president.

"She was polling when he was still in the race, so it was hypothetical. Now, people are gonna see her on TV every night against him," said one adviser to the Biden-Harris campaign who plans on advising Harris's team as the presidential nominee. "

"It was horrible, but that was then. This is now," the person added of her 2020 campaign. "She learned how to work with Joe Biden and win. She beat Donald Trump."

And perhaps more than anything, she's a sitting vice president and a historic figure in her own right -- a resume that could intimidate other would-be replacements enough to stay on the sidelines.

Steve Phillips, a Harris supporter and Democratic donor, told ABC News he's hearing from other donors and is expecting a "surge in enthusiasm and giving."

"A lot of people aren't going to jump in. Nobody is going to sacrifice their political career to take on a Black woman who's earned the right to run," the advisor added. "If we were anointing some random member, female member of Congressional Black Caucus, people would have a right."

Already, Harris is racking up endorsements.

Beyond Biden, she's nabbed the support of Bill and Hillary Clinton, the Congressional Black Caucus and Emily's List, the Democratic outside group and financial colossus.

She's also scored the endorsements of North Carolina Gov. Roy Cooper, Maryland Gov. Wes Moore, California Gov. Gavin Newsom and Pennsylvania Gov. Josh Shapiro, four lawmakers who were mentioned as possible Biden replacements, and dozens of current and former Democratic National Committee members are circulating an open letter to garner support for Harris.

"The best path forward for the Democratic Party is to quickly unite behind Vice President Harris and refocus on winning the presidency," Shapiro said. "I will do everything I can to help elect Kamala Harris as the 47th President of the United States."

Not every Democrat is sold that she can beat Trump, though.

Republicans are dumping millions into making Harris the poster child for the surge of undocumented border crossings -- an issue that polls show is one of the more significant policy matters in voters' minds.

A viral interview with NBC News in 2021 about her not visiting the border is already a centerpiece of the Trump campaign's opening attack ad against her, spliced together with clips of what Republicans have long said is an exaggerated laugh.

And her 2020 presidential campaign ended before any primary votes were cast, done in by severe dysfunction and a meandering search for policy stances that saw her take contradictory stances at times, including on healthcare and immigration.

Along the way, she also made some enemies of Biden's allies by attacking him over his stance on federally mandated school busing before struggling to come up with a policy of her own that was markedly different.

"I don't think she's authentic," said Democratic fundraiser John Morgan, who said he won't raise money for Harris. "All politicians are phony. Let's start with that. But some seem really sincere, and some seem like what I call fake phony, and fake phony is the worst phony."

"Not only was it not good, it was horrible," Morgan added of her 2020 race. "And she f----- over Joe Biden, and that's why I'm never going to be for her."

Seemingly sensing disquiet over Harris' frontrunner status, West Virginia Sen. Joe Manchin, a Democrat-turned-independent, is now considering re-registering as a Democrat to run against Harris for the presidential nomination, a source familiar with his thinking confirmed to ABC News.

Notably, other Democratic headliners, including former President Barack Obama, former House Speaker Nancy Pelosi and Senate Majority Leader Chuck Schumer, also didn't endorse Harris in statements praising Biden.

And it's not even technically a done deal that Harris would be the nominee -- party members are throwing around proposals for a mini-primary of sorts, and it's still possibly someone comes in with a head of steam and supplants her, although many of her would-be rivals quickly endorsed her."

But at the end of the day, Democrats took a big exhale Sunday, arguing that Harris is likely a stronger candidate against an ascendant Trump than Biden -- jumping over a bar that had been progressively lowered.

"I think most people would say that Trump has an advantage right now, but this is an election that's taken a lot of twists and turns already, so I think it's hard to predict the future," one Democratic strategist said. "But Vice President Harris gives us the kind of fighting chance we haven't had for the last three weeks."