Vice President Kamala Harris is the top alternative to replace U.S. President Joe Biden if he decides not to continue his reelection campaign, according to seven senior sources at the Biden campaign, the White House and the Democratic National Committee with knowledge of current discussions on the topic.

Biden's fumbling, sometimes-incoherent and widely panned first-debate performance against Republican rival Donald Trump last week set off a wave of panic within the Democratic party over concerns that he may not be fit enough to serve a second term and prompted calls for top aides to resign.

Some influential Democrats have floated alternatives to Biden besides Harris, including popular cabinet members and Democratic governors like Gavin Newsom from California, Gretchen Whitmer of Michigan and Josh Shapiro of Pennsylvania. But trying to sidestep Harris is wishful thinking and would be nearly impossible, these sources, who did not wish to be named, said.

If named as the party nominee, Harris, 59, would take over money raised by the Biden campaign and inherit campaign infrastructure, the sources said. She also has the highest name recognition among all the alternatives and the highest polling among Democrats who could seriously be considered a candidate, the sources said.

In a Reuters/Ipsos poll published Tuesday, Harris trailed Trump by one percentage point at 42% to 43%, a difference that was well within the poll's 3.5 percentage point margin of error, a showing statistically just as strong as Biden's.

In addition, she has already been vetted for national office and has survived intense scrutiny from Republicans, they said. Also, U.S. Representative Jim Clyburn, the man who was key to Biden's 2020 win, told MSNBC he would support Harris to be the Democratic nominee if Biden stepped aside.

"It's pretty near impossible to win the nomination over the vice president," said Michael Trujillo, a Democratic strategist from California who worked for Hillary Clinton's campaign in 2008 and 2016.

White House Press Secretary Karine Jean-Pierre said on Tuesday that Biden just had a "bad night" at the debate and would continue to make his case for reelection to the American people. The Biden campaign deferred to Harris's team for comment on the story.

Harris's aides dismissed any talk of a Democratic ticket that doesn't include both Biden and Harris. "Vice President Harris looks forward to serving a second term with President Joe Biden," a statement from her office said.

Biden's campaign has amassed 3,894 delegates after state primaries, leaving only a few dozen "uncommitted" delegates outstanding. They are expected to formally nominate Biden later this month in a virtual meeting, ahead of the Democrats' nominating convention in August.

"All of the delegates are not just Joe Biden delegates, they are Kamala Harris delegates," Trujillo said, adding "she will have a sizable delegation and support in all 50 states on day one," he said.

Donna Brazile, the former interim chair of the Democratic National Committee, who has a key committee role at this year's Democratic National Convention in August, said the person who can step in right away, if Biden decides not to run, is Harris.

"People may have dreams of another superhero but there is a process and the last time I checked it's a Biden-Harris ticket, she's number two on the ticket," Brazile said, adding Biden remains the nominee for the Democratic party and "is not going anywhere."

Passing over the first Black and woman vice president for another candidate would lead to a backlash from Black and women voters who are key to any victory, several Democratic strategists said.

'IMPOSSIBLE TO IGNORE KAMALA'

Still, Harris has been sidestepped in a lot of the speculation since the debate because some influential Democrats have little faith she can beat Trump, four of the sources said.

The United States has never elected a woman president, and Harris has spent much of her time as vice president struggling to distinguish herself in a role that is by definition a supporting one. As recently as last year, many inside the White House and the Biden campaign privately worried she was a liability for the campaign.

Since then, Harris has managed to find her stride on the issue of abortion rights but her polling has not improved significantly. Harris's approval ratings hover under 40%, but according to recent polling highlighted by the Biden campaign, she and the president have similar odds of beating Trump.

The vice president has also been consistently targeted by Republicans and conservative media in attacks many allies consider sexist and racist.

Three Democratic donors, who have recently been pushing for Biden to step aside, also said this week they think it will be "impossible" to sidestep Harris. The donors had been floating Whitmer and Newsom's names as possible alternatives until the past weekend.

"There is a real conversation in the Democratic party about leadership right now, but fair to say, and I'm not thrilled about this... it will be impossible to ignore Kamala," said one of the donors.

Another donor said "she's nobody's choice, but yeah, nearly impossible."

Still, the president's reelection campaign is standing its ground, emboldened by a stronger performance by Biden during a scripted speech in North Carolina even as calls for him to step aside grow.

Stephanie Cutter, deputy campaign manager for former President Barack Obama, whose firm is on contract to produce the Democratic National Convention in August, said "President Biden is the nominee and he's going to remain the nominee."

"For those who are looking for some sort of interparty fight, be careful what you wish for because that would ensure a Trump victory," she said in a statement.