(CNN) Former President Barack Obama, nearly a decade removed from last time his name was on the ballot, is about to face a key test to his clout and popularity inside the Democratic Party.

The comment was a not-so-subtle acknowledgment that this moment, where the Democratic Party hopes to come together after a bruising primary campaign, is exactly what Obama had waited for as he publicly stayed out of the nomination fight.

The election will be won or lost by Biden, not Obama. But Democrats across the ideological spectrum see the former president as the most powerful surrogate in the party's arsenal and Obama advisers have told top Democrats for months that he sees himself as the best positioned person to bring the party together once a nominee emerges.

That moment is now here after Vermont Sen. Bernie Sanders exited the race last week and endorsed Biden on Tuesday. But Obama's ability to be that bridge is questionable: Many in Sanders' movement, one partly made up of disaffected Democrats who see the former president as someone who squandered what he built in 2008, aren't moved by Obama.

"(This moment) will be kind of a test of the Obama appeal and legacy," said Evan Weber, the co-founder and political director of the Sunrise Movement, an influential liberal organization focused on climate change.

Weber, who like Obama was raised in Hawaii, said he was partly inspired to get into politics because of the then-senator's 2008 campaign, which centered on the ability of a large group of people to spark change and inspire hope. Like many in liberal circles, Weber said his most lasting memories of the Obama administration are when he and other activists worked with more progressive leaders like Sanders and Massachusetts Sen. Elizabeth Warren to oppose the president, especially on combatting climate change, handling the fallout from the financial crisis and entering into trade deals with foreign leaders.

"There are a lot of people that have a complicated relationship with Obama's presidency," Weber said. "And there is a whole new generation of the youngest voters who were just inspired by Bernie's campaign this cycle who don't have a lot of political memory of the Obama years and I don't know to what extent he will be able to inspire those folks."

The vast majority of Sanders supporters -- save those on Twitter who represent a vocal minority -- still acknowledge the impact that Obama's presidency made and most are likely to back Biden in November. But Weber's point of view highlights how the sway Obama safeguarded for months by publicly staying out of the race could be less powerful than he or his team believed it would be.

Obama, clearly aware of this skepticism, lauded Sanders in his lengthy straight-to-camera video endorsement of Biden. The former president called Sanders "an American original" and while he acknowledged they "haven't always agreed on everything," he knows that "the energy and enthusiasm he inspired, especially in young people, will be critical in moving America in a direction of progress and hope."

'Those differences pale'

People close to Obama, like longtime adviser Valerie Jarrett, believe that even with voters who have qualms with certain aspects of the Obama legacy, the former president is a powerful force.

"He enjoyed support from a broad spectrum of the Democratic Party, both the progressive and moderate wings, as well as independent and Republicans," said Jarrett, who acknowledged that winning over skeptical liberals and conservatives won't be solely Obama's job.

"Yes, there might be differences of opinion among the Democratic Party, but those differences pale," Jarrett added. "And for the greater good, we cannot afford four more years of President Trump. Will he get every single supporter of Sen. Sanders or all the other candidates? No. But is he uniquely positioned as the most popular person in Democratic party to be helpful? Yes."

But even former Obama advisers -- some of whom still talk to the former President -- believe his reach has its limits, especially with Sanders supporters who don't view his presidency with the same kind of nostalgia that older, more moderate voters do.

"Obama might help bring together some of the people who voted for Bernie and feel lukewarm on Biden, but I don't think he's the voice to bring the activists onboard. They don't see him as part of the movement," a former Obama adviser said. "Where I think Obama can be most helpful is with the infrequent Democratic voters and making sure they will vote in November."

Obama rarely spoke out during the 2020 primary, but in the brief moments that he did, the former president often lauded the excitement young organizers brought to the campaign, while also cautioning them from pushing the party too far out of step with most Americans.

"Even as we push the envelope and we are bold in our vision, we also have to be rooted in reality and the fact that voters including Democratic voters and certainly persuadable independents or even moderate Republicans are not driven by the same views that are reflected on certain left leaning Twitter feeds, or the activist win of our party," Obama told a room of top Democratic donors in November.

He added: "This is still a country that is less revolutionary than it is interested in improvement."

Liberal organizers saw those as an indictment of their work, and further widened the gap between the former president and liberal activists.

'Here is the big difference: Donald Trump'

It's unclear how Obama will be deployed during the 2020 campaign. In-person events have been halted due to the coronavirus pandemic, moving everything from fundraising to organizing online.

Obama has been an active surrogate in past elections, though. He stumped regularly for Hillary Clinton in 2016, including on the fundraising circuit, and followed the lead of a range of Democratic committees in 2018 as he hit the campaign trail to help his party take back the House. Obama has said for months that he will do whatever it takes to put a Democrat in the White House in 2020 and a source close to the former president said that he is prepared to take his cues from the Biden campaign on where he should be deployed and how he could be best used.

Obama's legacy of campaigning while his name isn't on the ballot is mixed, though.

Democrats took a "shellacking" in the 2010 midterms, with Republicans taking back the House and shrinking the Democratic majority in the Senate. The same happened in 2014, when Republicans kept control of the House and swept into the Senate majority.

And then there is 2016, where Obama forcefully endorsed Clinton and campaigned for her regularly, including the night before the she crushingly lost the election to Trump.

But 2020 is an entirely different election to Democratic activists. Trump has been president for four years, a tumultuous period where the Republican President, not a top liberal leader, has become the most unifying figure for Democratic voters.

"Here is the big difference: Donald Trump," Terry McAuliffe, the former governor of Virginia, said of the campaign in March, before Obama endorsed. "It is different."

Weber agreed, adding that "Trump definitely changes the calculation."

"There are lots of people who maybe sat it out in 2016 or voted third party," he said. "For many of those people, Trump at the top of the ticket will change that calculation. The question is how much."