Philadelphia (CNN) Sen. Elizabeth Warren and three other Democratic primary candidates will take center stage at the 14th annual Netroots Nation conference on Saturday, facing questions at a forum capping off days of activist workshops, panel discussions and a few morning yoga sessions.

Thousands of progressive organizers have been gathered here since Thursday, meeting to trade tips and tactics for building on last year's gains ahead of the 2020 elections . The Massachusetts Democrat will be the de facto headliner. Other early polling leaders are absent due to scheduling conflicts in what has already become, with nearly seven months to go before the Iowa caucuses, an unrelenting campaign for the party's nomination.

The presidential forum will also include New York Sen. Kirsten Gillibrand, former Housing and Urban Development Secretary Julián Castro and Washington Gov. Jay Inslee.

Netroots Nation, which debuted under a different title the summer before the 2006 midterms, is one of the longest running and most historically influential annual progressive political conferences, welcoming presidential candidates and top Democratic congressional leaders. Well before Twitter overtook blogs as the grassroots' digital megaphone of choice, it was instrumental in pushing party establishment leaders to more aggressively battle Republicans and embrace issues like climate change.

The Netroots model has become popular across the progressive political sphere, paving the way for dozens of similar events across the country and filling candidates' calendars as competing groups seek to elevate their priorities.

But with candidates like former Vice President Joe Biden, Sens. Bernie Sanders of Vermont and Kamala Harris of California, and Mayor Pete Buttigieg of South Bend, Indiana, all previously engaged, the 2019 edition of Netroots is less of a cattle call than a political trade show, with the party's activist base comparing notes and showing off their new wisdom following a 2018 midterm season that spawned a new generation of progressive elected leaders.

Three of those high-profile national newcomers, Reps. Rashida Tlaib of Michigan, Ilhan Omar of Minnesota and Ayanna Pressley of Massachusetts are making appearances; fellow "squad" member, Rep. Alexandria Ocasio-Cortez of New York is not slated to attend.

Avoiding a face-off?

True to form, the occasionally combative setting -- where mid-panel protests are more of a rule than exception -- has seen at least one serial squabble renewed ahead of the presidential forum. Daily Kos founder and co-moderator Markos Moulitsas suggested Wednesday on Twitter that Sanders, whom he has criticized in the past, is sitting out Saturday afternoon's question-and-answer session in order avoid a face-off.

In an email Thursday, Daily Kos communications director Carolyn Fiddler said a different kind of confrontation -- like the one Sanders and Martin O'Malley faced at Netroots in 2015 when they were interrupted by Black Lives Matters protesters -- might have deterred the Vermont independent from attending.

"I can't know what was on the campaign's mind, but it seems reasonable that his previous inability to handle a protest gracefully at this conference might be contributing to his nervousness about facing the Netroots again," Fiddler said.

Sanders aides have been consistently insistent that it was their schedule, and not any rumored angst over facing demonstrators or sharing a stage with Moulitsas, that would be keeping him away.

Asked about the other notable no-shows, Fiddler expressed some frustration that leading presidential candidates would pass up the chance to address Netroots' famously exuberant audiences.

"It's disappointing that Biden, Harris, and (Buttigieg) aren't attending, too," she said, "and are missing the opportunity to address over 3,500 of the most engaged, active members of the Democratic Party's base."

Moulitsas, whose group first convened the annual event in the mid 2000s, drew attention to Sanders' absence earlier this week, when he shared a Daily Beast report claiming that his involvement in the candidate forum "raised concerns" with the Sanders campaign.

"Bernie Sanders goes on Fox, but he's afraid of ME?," Moulitsas wrote, alluding to the Vermont independent's Fox News town hall in April. "Sheesh."

Though Sanders won't be in Philadelphia, his campaign will not go unrepresented. On Thursday, campaign co-chair Nina Turner joined protesters angry over the closure of the city's Hahnemann University Hospital. She also sat on a Friday panel called, "Black, Brown and in charge: New movement leaders and the future of the left," alongside the Working Families Party national director Maurice Mitchell, Democracy for America CEO Yvette Simpson and Karundi Williams, executive director of re:power.

Tracey Corder, the director of federal action and racial justice at the Center for Popular Democracy, said that Netroots' longevity is a testament to its unifying mission.

"The progressive movement is made up of pockets across the country that are doing amazing work in community and on the ground," Corder said. "Netroots allows grassroots activists to gather in fellowship face-to-face. These bonds are powerful and they are the force that sustains Netroots."

Corder told CNN she didn't regard the slender slate of presidential hopefuls as an affront, but warned that other devoted attendees would take notice.

"It's less about my personal feelings about candidates being at Netroots," she said. "It's about the signal they are sending about how they value the progressive arm of the party when they don't show up."