Buzz builds around Kamala Harris should Biden withdraw

President Joe Biden and Vice President Kamala Harris met for lunch on Wednesday -- likely their first one-on-one interaction since the president's lackluster debate performance . However, unlike previous meetings, this one comes as momentum builds for Harris to replace Biden at the top of the ticket with new polling showing her outperforming Biden in a hypothetical matchup with former President Donald Trump.

While most prominent Democrats kept mum in the immediate aftermath of the debate or issued short statements of support, early this week, several current and former party elected officials called on the president to withdraw from the race . Some of Biden's allies even pushed him to make unscripted appearances and answer tough questions from reporters to show his fitness for office.

Sources told ABC News on Wednesday that Biden has privately acknowledged that the next few days are critical to determining whether he can stay in the race for a second term -- something the White House denied.

Harris has pledged her full support for Biden since the president's first debate this cycle with Trump.

On ABC News Live the night of the debate, Harris defended Biden's performance, saying "the bottom line is this, let us not decide who is going to be the president of the United States based on the 90-minute debate. Let's measure that decision against the last three and a half years." And in an interview with CBS News on Tuesday, she didn't mince words: "Look, Joe Biden is our nominee."

Harris joined Biden on an all-staff campaign call on Wednesday, during which Biden was "unequivocal" that he was staying in the race, several sources familiar confirmed to ABC News.

Yet despite Harris' repeated defenses of Biden, pressure on the president is building -- and the vice president has earned the support of many Democrats who see her as a logical successor if Biden were to withdraw.

Former Ohio congressman and 2020 Democratic presidential candidate Tim Ryan wrote in an opinion piece published Monday night in Newsweek that Biden should step aside so that Harris can become the Democratic nominee. Biden "promised to be a bridge President to the next generation," wrote Ryan, but "that bridge collapsed last week."

Democratic Rep. Jim Clyburn -- a former House whip and co-chair of the Biden campaign -- said on MSNBC on Tuesday that he would support Harris taking over the top spot on the ticket if Biden were to "step aside."

"No, this party should not in any way do anything to work around Ms. Harris," said Clyburn, in response to speculation that the party could choose a different replacement. "We should do everything we can to bolster her -- whether it's in second place or at the top of the ticket."

Former Housing and Urban Development Secretary Julian Castro on Tuesday called for the president to withdraw and pointed to Harris as a strong option. He also noted that "we have a stable of folks that I think could do a better job" than Biden.

A new CNN poll is fueling some of the Harris hype. It shows the vice president just two points behind Trump in a hypothetical head-to-head matchup -- within the margin of error. Biden, by contrast, trails by six points, and no other Democratic replacement polled fares as well as Harris.

Harris' lead in the CNN poll compared with other rumored Biden alternatives such as Michigan Gov. Gretchen Whitmer and California Gov. Gavin Newsom underscores one key advantage the vice president has over other Democrats: national exposure. In the event of a late switch, Harris would likely have less work to do to define herself to the broader American public. And she has already been subject to painstaking vetting by the national press and her opponents.

More concretely, because the vice president already shares the presidential ticket, she can directly inherit most of the campaign's war chest -- more than $91 million the Biden campaign has in cash-on-hand. Other candidates, by contrast, would face difficulties claiming the funds. The funds give her a major advantage over any other potential replacements.

The newfound momentum behind Harris comes despite months of poor polling. In 538's polling average , just 37.4% of Americans approved of the vice president's performance at the start of the year -- roughly 2% fewer than approved of Biden's performance , according to 538's polling averages.

Harris' approval rating remains lower than Biden's in the latest CNN poll, 29% to 34%, respectively.

Republicans, who have previously attacked Harris as "woke," have already trained their sights on the vice president as the election approaches.

A new digital ad from the National Republican Congressional Committee out Wednesday attacked Harris over the administration's border policy, framing her as Biden's "border czar." Another recent Trump ad shows a montage of Biden's gaffes and stumbles -- and warns that Harris is poised to take over: "Vote Joe Biden today, get Kamala Harris tomorrow."

While the administration initially tasked her with the politically difficult issue of immigration, Harris -- the first Black vice president and the first woman vice president -- has more recently served as the lead messenger on racial justice and the central campaign theme of abortion rights.

Whether Harris will move to the top of the ticket largely depends on Biden. Only he can decide whether to release his pledged delegates -- or throw his support behind his vice president -- ahead of the Democratic convention in August.