Progressives to test 2020 Democrats' leftward pull at NetRoots Nation conference

In the early stages of a 2020 Democratic primary brimming with presidential candidates seeking to outflank each other from the left, some in the field jockeying for the progressive mantle will test their bold ideas before an eager left-leaning base at the NetRoots Nation annual conference this weekend.

Along with Massachusetts Sen. Elizabeth Warren, a top-tier candidate in recent polling, former Housing and Urban Development Secretary Julián Castro, New York Sen. Kirsten Gillibrand, and Washington Gov. Jay Inslee are converging on Philadelphia for the gathering of liberal activists, which has a reputation for channeling progressive energy while pushing lawmakers and candidates to the left on a range of issues.

The last time presidential candidates descended on the annual conference was during the 2016 presidential contest, when runner-up Bernie Sanders’ status as the lone outsider of the progressive wing elevated his populist platform as a foil against the ultimate nominee, Hillary Clinton. Despite moving the party leftward four years ago, the Vermont senator is no longer the progressive standard-bearer in 2020, but instead standing among a parade of contenders, including his friendly rival, Warren, who broadly shares his vision for economic equality.

In 2015, both Sanders and another competitor, former Maryland Gov. Martin O'Malley, were met by disruptions and heckling crowds over the emerging Black Lives Matter movement at the NetRoots conference. O’Malley exited the stage amid boos from the audience, and this year, Sanders is reportedly skipping the event entirely.

For decades, Democratic circles have had a longstanding debate over what defines a "progressive," but that conversation saw renewed urgency amid Sanders' unexpectedly competitive challenge to Clinton in 2016.

The two-time presidential candidate’s 2020 policy platform is nearly identical to his 2016 agenda, which emphasizes universal health care and tuition-free public universities as the cornerstones of his campaign. But this time around, many of his ideas are seeing broad support among the rest of the field -- in part due to the man currently occupying the White House.

"[Sanders] actually helped move the party and the party has changed since then, and the context of [President Donald] Trump gives Democrats a little more freedom to go into new places," Julian Zelizer, a professor of history and public affairs at Princeton University, told ABC News in April.

"In some ways the unconventional, unpredictable and -- some would say -- radical aspects of the Trump presidency, have made some Democrats more willing to say, 'Why do we have to play it safe all the time? Why not be bold?'" Zelizer added.

Those competing in the presidential contest this cycle, many of whom continue to bandy with the term progressive and its myriad of meanings, will appear before some of the most leftist Democrats in the base on Saturday. Before the candidates are set to take the stage, some of the progressive activists that will be in the crowd point to nuanced distinctions in the candidates themselves to determine who is "progressive" -- particularly Sanders and Warren, the two progressive stalwarts -- as they continue to mull who to back to ultimately compete against Trump.

Several attendees at the three-day conference, most of whom work for progressive organizing and activist groups comprising the left flank, said the difference between Warren and Sanders, despite their similarities in ideology or policy, is the former Harvard professor’s outsider status, which makes her the more "practical progressive" who can beat Trump.

"There are only two people in the current field that fit that definition," Brent Simmons, 70, a former civil rights attorney with the NAACP, Legal Defense and Educational Fund, told ABC News Friday. "And that's that's going to be Bernie Sanders, obviously, but I support Elizabeth Warren as probably the most practical progressive in the field."

Among this cohort of the progressive wing of the party, a number of attendees acknowledge Sanders’ influence on pushing the party to the left and bringing liberal issues to the forefront of national politics, but they also still single out Warren as the one who can win.

"Bernie Sanders, I certainly credit him with setting the agenda, but I think Elizabeth Warren has a better chance of being elected between those two," Simmons said.

"I give Bernie a lot of credit for bringing so many of these progressive values and vision for the country to the forefront in 2016," said Nick Guthman, the son of union organizers who launched Blue Future, a political action committee (PAC) focused on engaging young people in the electoral process. "Obviously, many of his ideas are being put front and center now in the presidential campaign for 2020. ... I particularly like Elizabeth Warren, I think she's putting forward plans and vision for the country that is truly progressive, about also recognizing the realities that we face in our system, both in capitalism and democracy."

"I think when it comes down to actual policies, she has better plans that are more detailed … more thought-out than Bernie Sanders," he added.

"I see Bernie as a career politician. I see him as not a Democrat. I think he's an Independent. And I see Elizabeth Warren as a teacher. Her career has been in teaching. She stepped into politics because she saw a need," said Virginia Harris, a South Philadelphia resident who works in the clean air industry and as a historical author.

But amid the consistent refrains of support for Warren and/or Sanders, over the course of the early months of the primary, several Democrats in the field have claimed the progressive mantle, including former Vice President Joe Biden.

"The definition of progressive now seems to be changing," Biden said in late March when pressed about a previous comment he made on having the most progressive record among the field of 2020 presidential candidates. "It's, 'Are you a socialist? That's a real progressive.' ... I'll stack my position against anybody who has ever run or is running now or who will run."

But in the broader debate about progressivism within the party -- which one attendee at NetRoots said is now an "umbrella term" after so many Democrats used the label -- the epicenter of the conversation among the far-left segment still lies within the divide between Sanders and Warren -- showing how much the liberal base has changed since 2016.

As Franco Caliz, an attendee with the advocacy organization Community Change Action, parsed through the 2020 field, he shared a philosophy that echoed throughout the Philadelphia Convention Center: while there are "quite of few" progressives in the field, Warren is at the top and captured broad support from these progressive activists.

"Biden shows a lot of his voting record comes from the '90s. I think the political climate is shifted pretty dramatically. That’s not the politics that are currently in vogue," he said Friday.

"For me, one of the keys is that Sen. Sanders describes himself as a Democratic socialist and is a socialist pretty proudly, and coming from Nicaragua, a country that has a pretty lengthy history with that particular ideology, [I like] having someone like Sen. Warren, who believes in regulated free markets but free markets nonetheless. ... Capitalism is pretty significant and a much stronger way of reaching out to certain pockets of the Latino community," he continued.

ABC News’ John Verhovek and MaryAlice Parks contributed to this report.