The first Netroots Nation convention was 13 years ago, with about 600 followers of the progressive website Daily Kos agreeing to meet at the Riviera Hotel in Las Vegas.

“It was basically started as this meeting for people who used the website,” said Adam Bonin, an election lawyer in Philadelphia. “Almost as many reporters as attendees showed up that first year.”

As the annual convention has grown, so has its influence. No longer seen as fringe or extreme, the progressive movement has become a key voice in the battle for the direction of the Democratic Party — and the selection of its 2020 presidential nominee. This year’s convention, which opens Thursday in Philadelphia, is expected to draw 3,000 people, culminating Saturday in a forum for presidential candidates at the Convention Center.

“Folks who come to Netroots aren’t just vocal," said Mary Rickles, political director for the group. “They’re the base. They knock on doors. They make phone calls. They work for organizations that put tons of money into these elections, so it’s not just like 3,000 people who come to the conference and then go home. It’s the thousands of people who work for them, and that they’ll touch.”

Many of the large progressive groups represented at Netroots are either explicitly or more subtly against the party’s front-runner, former Vice President Joe Biden, and argue that a more liberal candidate who can excite younger Democrats to turn out deserves the nomination. The panel topics during the convention will echo issues candidates debated in June.

“To see issues we championed for years, like Medicare for All and climate change, so prominent on a mainstream Democratic debate stage is really refreshing, because we’ve been at this work for so long,” said Yvette Simpson, who heads Democracy for America, one of the longest-running progressive political action committees, founded in 2004. “We see a lot more candidates who are talking more boldly and unapologetically about progressive values, and I think what we’re seeing is those who don’t receiving a little bit of backlash.”

With more than two dozen declared candidates, Netroots limited its presidential forum invitations in order to give each person 20 minutes to answer questions posed by moderators and the audience. Saturday’s event is expected to draw Massachusetts Sen. Elizabeth Warren, former Housing and Urban Development Secretary Julian Castro, New York Sen. Kirsten Gillibrand, and Washington Gov. Jay Inslee.

The event will also showcase some of the movement’s more prominent rising voices. Hours before the presidential candidates forum, U.S. Reps. Ilhan Omar (D., Minn.), Deb Haaland (D., N.M.), and Rashida Tlaib (D., Mich.) will address attendees. Philadelphia District Attorney Larry Krasner and Councilwoman Helen Gym are also scheduled to speak.

Though many share policy views, the event could be an opportunity for the lower-polling presidential candidates — like Inslee and Gillibrand — to stand out to a key constituency.

“Progressive is the new black," said Simpson, of Democracy for America. “We are thrilled to see so many candidates representing bold, unapologetic, progressive values, but it’s almost come to the point where we need a litmus test for that."

Sen. Bernie Sanders, the Vermont “democratic socialist” whose 2016 run paved the way for many of the ideas being talked about this time around, was invited to the Philadelphia gathering but won’t attend. In 2015, he and fellow presidential candidate Martin O’Malley were heckled at a similar forum in Phoenix by protesters of police brutality. Both candidates seemed to stumble in their answers.

Sanders’ campaign said this year it would be sending a surrogate in his place. Another candidate who has been drawing attention, Sen. Kamala Harris of California, also won’t be there, choosing instead to address a national convention of a black service fraternity in Atlantic City.

Biden was also invited, but will be campaigning in New Hampshire this weekend. In 2014 Biden spoke about marriage equality at the Netroots Convention.

A term coined in the early 2000s, netroots referred to grassroots political activism organized through online media. In the beginning, the movement wasn’t as leftward leaning, drawing such establishment stalwarts as House Speaker Nancy Pelosi and former Senate Majority Leader Harry Reid to its first convention.

But it has shifted along with the party, much the way conservative Republicans have pushed the edge of their party further to the right. Sanders’ unexpected popularity three years ago signaled an appetite for that shift, said Sarah Niebler, a political science professor at Dickinson College.

“There really weren’t any negative repercussions for Sanders in 2016, which, I think, emboldened the current candidates of 2020 to adopt more liberal positions,” she said.

Niebler said she’s sensed a tendency to equate more moderate candidates with the establishment. And experience isn’t necessarily a selling point, thus some of the progressives’ aversion to Biden.

“We’re in an era where trust in government is at an all-time low, so there’s a strong desire to run against the government," Niebler said. “What better way is there to do that than to propose policies the government has been unable to move past?"

Republicans have already used the leftward movement and policy proposals like Medicare for All and the Green New Deal as a way to cast next year’s election as freedom vs. socialism. But polls show that a number of Democrats, particularly younger ones, aren’t bothered by more socialist policies, and that Democrats who identify as liberal outnumber those who identify as moderate.

As the party grows, that voter base will, too, said Sean McElwee, founder of Data for Progress, a liberal think tank.

“Today, a democratic socialist is what you call a Democrat under 40 in a city with term limits,” McElwee said. “And regardless of whether or not younger Democratic politicians identify as democratic socialists, they are definitely more liberal. So, the progressive movement, it has way more power than it had two years ago, and way less than it will have in two years.”

How candidates are received at the convention, McElwee said, is a good indicator of how they’ll do down the road.

“It’s a chance," he said, "to get out in front of the activist space that is influential and that is a leading indicator of where the rest of the party will go.”