PHILADELPHIA — If there were a summer camp for the "vast left-wing conspiracy," as Trump White House adviser Kellyanne Conway has dubbed it, it'd be the annual Netroots Nation Conference here this weekend.

And among those campers — the 3,700 liberal activists and operatives — two candidates in the upcoming Democratic presidential primary are clear favorites: Sens. Elizabeth Warren, D-Mass., and Bernie Sanders, I-Vt.

But only one of the two showed up to address the faithful. Warren received a rockstar welcome at a candidate forum Saturday, while Sanders skipped the conference and sent his campaign co-chair instead.

The Sanders campaign told organizers last week that he could not attend due to a scheduling conflict. Other candidates also in the hunt for progressive endorsements, such as Sens. Kamala Harris, D-Calif., and Cory Booker, D-N.J., missed the gathering as well.

"A movement candidate comes to Netroots and engages with the people who are doing the work not just to speak, but to listen to the experts," said Jennifer Epps-Addison, the president and co-executive director of the Center for Popular Democracy Action, a network of local progressive groups.

Still, with Warren and Sanders standing above the rest, his absence and her presence appear indicative of their larger approach to the progressive movement — and, some say, help explain why she's been climbing in recent polls while his support has remained flat.

Where Warren has long partnered with left-wing groups to coordinate her work inside the Senate with their pressure from the outside, Sanders has often preferred to go it alone and build his own movement. It's an approach he's taken not only with the Democratic Party but also with some of the established progressive groups that come to conferences like this and have an outsize voice in the primary process.

"Warren has been coming here for the better part of a decade," said Markos Moulitsas, the founder of liberal blog Daily Kos, from which the Netroots Nation conference spun off. "There's a long history of her engagement with the Netroots that's really paying off."

When she stepped onto the stage Saturday, the familiar crowd greeted her with a lengthy standing ovation that included chants of her name and shouts of "We love you Liz!" It was a markedly more passionate response than that given to the other three candidates present, Kirsten Gillibrand, Jay Inslee, and Julián Castro.

While Warren has invested in the basic personal care-and-feeding chores of politics, such as sending thank you notes and making phone calls to allies on their birthdays, Sanders tends to chafe at the kind of ego-stroking it often takes to build and maintain relationships among insiders and influencers — especially if they haven't always been with him.

Sanders has built a loyal base of followers that gives him perhaps a sturdier floor of support than almost any candidate in the field, but Warren may have a higher ceiling thanks to her ability to straddle the establishment/anti-establishment divide.

Sanders last spoke at Netroots Nation in 2015, when he and fellow presidential candidate Martin O'Malley were repeatedly heckled and booed by Black Lives Matter activists. The conference has developed a reputation for disruptions like that, which has kept some high-profile speakers away.

But Moulitsas, who moderated Saturday's presidential forum, said it was a missed opportunity for Sanders to make his pitch about why he should be the one to carry out the vision of his 2016 campaign instead of passing the torch, especially to a woman or person of color, since "white men are now a minority in our party."

"It's remarkable how he mainstreamed progressivism in the Democratic Party. And he deserves a lot of credit for that — it doesn't mean he deserves the nomination for that," Moulitsas said. "So it's too bad he didn't come...I would have loved to publicly personally give him thanks for that."

He added, "But I've been saying that it requires a certain sense of arrogance to be a white man and think that this is your election cycle.”

Sanders consolidated support on the left back when he was the obvious — and only — progressive alternative to Hillary Clinton.Today, the movement is divided.

Sanders campaign co-chair Nina Turner, a former Ohio state senator who attended Netroots on his behalf, said it's "insulting" to suggest that "the most progressive and most consistently progressive" candidate should pass the torch now that his ideas are more mainstream.

"There are many copies in this race, but there’s only one original," Turner said. "Here’s the man that was in the trenches all his life. Here's the man who was bold enough to run in 2016 and take all the slings and arrows...and you mean to tell me that the man who stood up to the system needs to bow out? I don't think so."

Sanders, she said, works every day to build the progressive movement across the country and advance its agenda in Washington, like getting the Senate to vote against U.S. intervention in Yemen — so "he doesn’t have to wait for a once-a-year conference."

The so-called netroots, which grew out of a different era of "resistance" under President George W. Bush, hardly represents the entirety of the progressive movement today. And many have been behind Warren since even before Sanders ran the first time, hoping to draft her into the 2016 race before switching to Sanders when she stayed out.

Even here, there are plenty of people who argue Sanders is the only candidate who can be trusted to follow through on the vision he and Warren largely share. And some are still sore at Warren for declining to endorse Sanders in 2016.

"I support most of her economic policies, but Bernie's just better. He’s been consistent for so long," said Russell Cirincione, a Sanders supporter who is running an underdog primary campaign against New Jersey Democratic Rep. Frank Pallone. "Bernie's been setting the standard, everybody else is really playing catch up with him."

Leading liberal groups like MoveOn.org and Democracy for America backed Sanders in 2016 but are weighing the larger field of candidates now and have not yet thrown their support to anyone, with some actively considering numerous candidates including Sanders, Warren, Harris, Booker and former Housing Secretary Julián Castro.

Still, many, like Rep. Alexandria Ocasio-Cortez, D-N.Y., say they are mainly deciding between Sanders and Warren.

A new NBC News/Wall Street Journal poll this week found Warren had pulled into second place with 19 percent, behind former Vice President Joe Biden at 26 percent, leaving Sanders tied for third with Harris at 13 percent.

And Warren edged out Sanders' vaunted online fundraising machine in the second quarter of the year, raising $19 million to his $18 million — roughly the same amount he collected in the first quarter of the year — after the controversy over Warren's Native American heritage marred the start of her campaign.

Still, even while groups and left-wing thought leaders say they are deciding between the two, there are signs there may be less overlap between Sanders and Warren rank-and-file voters than commonly believed.

Some recent polls suggest Warren supporters are more likely to pick Harris as their second choice, while many Sanders backers opt for Biden over Warren, despite their ideological differences, if the Vermont senator were to drop out.

The sprawling 2020 primary is split along more than just ideological fault lines, but also differences of gender, race and style as well. And Warren's support tends to be whiter, older and more educated, while Sanders' is younger and less educated, polls show.

Either way, some on the left feel a sense of urgency to unite behind one candidate sooner than later to avoid splitting their vote and helping someone like Biden, who many on the left say lacks the ferocity and vision they feel is necessary.

For instance, the Working Families Party, which backed Sanders last time, is now actively considering six candidates ahead of a likely late summer or early fall endorsement, with Sanders and Warren leading internal surveys of members.

"Corporations have already started picking their candidates," Joe Dinkin, the group's national camapigns director. "If progressives don't organize quickly, Wall Street could pick the Democratic nominee for us."