President Joe Biden's aides privately acknowledge a rematch with Donald Trump in 2024 could be his best path to reelection, but they are terrified by what would happen if he lost -- which operatives on his campaign and throughout the Democratic Party worry is a very real possibility.

It's not just that voters continue to say Biden is too old or maybe not up to the job, though that keeps coming up in Democrats' focus groups. It's a malaise about the president that operatives keep noting that goes beyond a slew of national polls -- including one from CNN last week -- that show a negative view of the president's performance.

"I feel indifferent, honestly," said a voter in one late summer focus group conducted in a strong blue district, one of several described to CNN by operatives involved. "I don't have a very strong opinion, except I'm glad that it's not the previous person."

"I feel like he's done no harm versus the previous president," said another. "It's not bad, it's not great."

"It's basically just trying to bring our country back up to where it was before," said a third. "So instead of doing great things, just kind of keeping us more middle ground."

Several Biden aides themselves tell CNN that looking at Trump's wide lead in every primary poll -- and no clear leader in a hypothetical Biden-Trump rematch -- leaves them with a particular kind of existential stress. The former president may seem like the easier Republican candidate to beat, as a known quantity who is toxic to many voters, but they're terrified that if he is the nominee, any fumbles by the campaign or the president himself would enable the return of a candidate they see as a singular threat to American democracy.

Even as some prominent party leaders privately wonder if Biden should pull out of running, aides to the president point the blame in part at the media for what they view as validating concerns about Biden's age and about Republican claims of Hunter Biden's corruption by covering those concerns, despite what they argue is a lack of evidence. They note that Barack Obama's poll numbers were also weak going into his reelection campaign. And they are weighing what the possible political impact could be from the impeachment efforts House Republicans stepped up this week. They think it could backfire on the GOP, but it could also intensify the focus on the relationship between Biden and his son.

They still project confidence Biden will win -- but they acknowledge there is a very real chance that he could lose.

Leaning into the threat of what happens if he does lose will be a central part of the campaign.

"The president has said publicly himself that democracy is still on the ballot. If you believed that to be true the first time, you have to believe that to be more true the second time," said Jen O'Malley Dillon, a Biden White House deputy chief of staff and his 2020 campaign manager, who spoke to CNN in her personal capacity in order to discuss politics.

Or, as a different aide put it, discussing the lack of enthusiasm for Biden right now, "I don't need people sprinting to their polling places -- as long as they make it."

"Do their votes count differently?" asked another.

But aides are stepping up outreach to holdout major donors to help fund what one Biden senior adviser said "is going to require us to be the best campaign that has ever existed for the president, when we are going to be up against things we have never faced before."

Inside Biden's campaign headquarters -- where desks have been slowly filling up since the week before Labor Day, and in the bigger offices in the West Wing, where nearly every decision from staffing to strategy is still running through -- aides have not shifted from their plan to stay mostly under the radar and off the trail until next spring.

In the meantime, they're ramping up an extensive data and outreach experiment that they say is the only way to account for how much has changed in politics, technology and psychology in the eight years since the last presidential election not defined by social distancing, car rallies, Zoom fundraisers and the rest that came with the Covid-19 pandemic.

The mission: see which Republicans and swing voters they can pull back from drifting to Trump, and whether there is any hope of getting the wider electorate actually excited about anything from Biden.

The reelection campaign is laying the groundwork for an argument that goes beyond Trump in case they instead face one of the three or four GOP candidates they still see as having a chance. (Florida Gov. Ron DeSantis is still on their minds, with an outside chance for South Carolina Sen. Tim Scott or former UN Ambassador Nikki Haley.) The difference between 2024 and the last two elections, they will say, is that the internal Republican resistance to Trump and what Biden calls "MAGA extremism" has completely eroded.

"There's no real delta," added Biden campaign communications director Michael Tyler, surveying the Republican field on issues like government and abortion rights, "when you're talking about who would be sitting behind the Resolute Desk."

That is the benefit of building up "while letting the Republicans fight each other during a multi-MAGA headed brawl," said Jef Pollock, a veteran consultant and pollster not directly working with the campaign who has been urging fellow Democrats to calm down. "With most campaigns, you need to drive a narrative about the opponent. In this one, the opponent drives the narrative against himself quite well."

An autumn of unprecedented voter testing and experimentation

It's not just that media consumption is fragmenting more and more -- making traditional campaign tools like ads on broadcast TV increasingly irrelevant -- but that voters are rapidly shifting in what they trust of what they see and read.

"Normal," said Biden deputy campaign manager Rob Flaherty, who recently moved over from running the president's online presence at the White House, "has evolved."

The Biden campaign is collecting information like who is watching their early microtargeted online campaign ads and for how long, who is loading them up again, who is sharing them with their own social media networks and which networks. They are testing not just which voters the campaign is reaching and what the campaign is saying, but how the voters are being reached, how they're processing the information -- and whether any of that can be turned into votes. Do multiple ads make a difference? Does it work to send a follow up text to people who have seen ads? How long should they wait?

"This is a competitive election that came down to really fine margins last time," Flaherty said. "We would be crazy not to be experimenting."

While Republicans focus on states like Iowa and South Carolina -- which have early nominating contests but are not expected to be competitive in the general election -- the Biden campaign is building off an unprecedented database of voter information culled from hundreds of progressive groups and Democratic campaigns to micro-target and data mine in battlegrounds like Pennsylvania, Wisconsin, Arizona and Georgia. This will also be built into new avenues, on social media and in person, that the Biden campaign will be experimenting with to organize and connect with voters that go beyond the traditional volunteers knocking on doors with clipboards.

Testing how to turn voters off of Trump and on to Biden now, they argue, will give them an unmatchable head start to guide them through how to spend hundreds of millions in advertising and organizing next year.

"We're trying to run programs now that will mimic the kinds of things we want to do in 2024 -- that allows us unprecedented opportunity," Becca Siegel, Biden's former chief analytics officer recently re-hired as a senior adviser, told CNN.

As the Biden campaign convenes some of their bigger donors this week in Chicago for a convention preview-themed confab, they're trying to convince more of their richest supporters to believe more in the plan, and in Biden, and chip in with the early funding to pay for it.

Most expect the race to come down to a few hundred thousand votes in a few states, but Siegel said that the campaign is already tracking more fluidity than what early polls indicate.

"Whether that is movement from not caring at all to caring a lot, or moving them from Donald Trump to Joe Biden," Siegel argued, "there is a lot of movement.

Frustration with top tier of donors

Biden far outraised Trump and all the other Republicans combined for the quarter that ended over the summer, with help from many top Democratic donors. But to the exasperation of several top Biden aides who spoke to CNN, some of the most notable names and biggest checkbooks remain missing.

Those include former New York City Mayor Mike Bloomberg and investor and environmental activist Tom Steyer, who together spent nearly $1.5 billion in their failed presidential campaigns to take on Trump in 2020.

Howard Wolfson, who manages Bloomberg's political involvement, said it would be wrong to read not putting up any money so far into anything negative about Biden, explaining, "We don't really have those conversations internally until the election year." The wooing is very much underway, with White House chief of staff Jeff Zients speaking with Bloomberg directly just last week -- though they did not touch on political support directly.

As for Steyer, "Tom believes the Biden administration has been one of the best in history on climate, and he will be enthusiastically supporting President Biden's re-election campaign," said adviser Doug Rubin. Talks are underway for him to host Biden at an event at his home.

But Biden's struggles to convince even the non-multibillionaire top donors continue. Longtime Democratic megadonor Jeffrey Katzenberg is serving as campaign co-chair and trying to bring his usual campaign finance rainmaking in-house while former ambassador Rufus Gifford left the State Department for a finance job, traveling the country to make direct appeals. And aides have started arranging a series of one-on-one calls Biden has been making from the White House with major potential donors, after years of them feeling ignored and left out.

But Biden has never been as popular with the type of big donors who tend to like being up close with flashier, more celebrity-style candidates. These days, though, many are even less convinced by him, telling other Democratic politicians and operatives that they also are not inspired by the thought of him running, and they would rather be invested in candidates they see as more part of the future than the past.

Over three days this week in Chicago, which will be headlined by appearances from Vice President Kamala Harris, top Biden strategist Anita Dunn and campaign manager Julie Chávez Rodríguez, the reelection campaign will be trying to woo many of their other top donors, reminding them that polls this far out ahead of an election tend not to mean much, and that other early indicators like Democrats' over-performance in the 2022 midterms and multiple special elections are promising. They will make the case, as a White House aide put it to CNN, that the view remains to "focus on executing on the agenda and delivering results, and the politics will resolve."

But the Biden senior adviser added hopefully, "Everything, every day, feels a little more settled, a little more stable, a little in more place of comfort," and with more than a year of that to go before the election, "people are going to look up and eventually say -- that's because of the president."

Trump is the solution to fundraising too, said major donor John Morgan.

"At the end of the day, money is not going to be a problem," said the famous trial lawyer, who is still waiting to firm up a Florida event he has been talking with the Biden campaign about hosting since the spring. "The fear of Trump will trump that."