Bernie Sanders is surging in national polls and packing stadiums with tens of thousands of people in cities like Los Angeles, Seattle, Phoenix, and Madison, Wisconsin.

But as his political star rises, the long-shot 2016 presidential hopeful must contend with ever-more forceful—and frequently public—pressure from a flood of progressive activists working hard to grab a bit of Sanders' spotlight.

The self-styled Democratic socialist candidate's popularity only makes him an even more appealing target for anyone on the Left looking to shape the 2016 agenda.

"This is an opportunity for progressives to get issues that often get buried front and center," said Steve Rosenthal, a veteran progressive political strategist and former AFL-CIO political director currently working on state and local campaigns. "Organizations and causes on the Left view this as a chance to make sure that gets raised on the campaign trail."

That's unlikely to end as long as Sanders' populist appeal continues to resonate.

As a result, the Sanders campaign faces an early test. It is working to rally a critical mass of support in the 2016 race, and deliver a unifying message—voiced loudly enough for voters and Hillary Clinton to hear—even as it is pulled in different directions by the very movement that has fueled its rise.

The question is: Can Sanders convince a wide, and sometimes disparate, array of progressive activists that they should line up to support his campaign?

To hear the campaign tell it, the answer to that question is a resounding "yes."

"The things we're talking about in our campaign are the things that folks are talking about with their friends; these are the things folks are talking about at the dinner table," said Symone Sanders, the campaign's recently-hired national press secretary.

Symone Sanders, who is not related to the Vermont senator, is an outspoken criminal-justice advocate. The campaign announced that she had joined its ranks hours after the 2016 contender was shouted off stage in Seattle by activists claiming to be part of the Black Lives Matter movement.

"The senator is so focused on making sure we address economic inequality, racial inequality, that we are speaking to the dangers of climate change, that we are talking about the wage gap and investing in infrastructure—these are the things that are important to the American people, and I think that's why we have had such a warm reception and why we will continue to get such a warm reception," Sanders added.

But Sanders the candidate has not always gotten a warm reception. The confrontation with protesters on Saturday eventually forced him to leave the stage. And that wasn't the first time the campaign has clashed with the modern-day civil-rights protest movement.

In July, Black Lives Matter activists interrupted the regularly-scheduled programming at the progressive Netroots Nation conference in Phoenix, Arizona, demanding that Sanders and Democratic 2016 rival Martin O'Malley spell out how they would end police brutality.

Sanders got defensive. "I've spent 50 years of my life fighting for civil rights. But if you don't want me to be here, it's OK." he told the crowd, before launching into a recitation of what has long been his core message that America must fight income inequality.

Still, the Sanders campaign is working hard to respond to progressive pressure.

Over the weekend, the campaign rolled out a new section of its website devoted entirely to racial justice, a platform that quickly won praise from Black Lives Matter activists. At an event on Sunday, the candidate had the words to match.

"We know the truth that, like every community in this country, the vast majority of people of color are trying to work hard, play by the rules, and raise their children. It's time to stop demonizing minority communities," Sanders shouted from the stage.

Black Lives Matter is far from the only progressive group with a political demand that the Vermont senator will need to reckon with on the campaign trail. And the more that Sanders's popularity surges, the more of an incentive progressive activists have to disrupt—or at least try to shape—his campaign.

On Tuesday, Harvard professor Lawrence Lessig formally announced that he intends to run for president if none of the leading Democratic contenders prioritize campaign-finance reform as their No.1 agenda item.

Sanders has already made campaign-finance reform a key pillar of his 2016 platform. But that apparently wasn't enough for Lessig, whose entry into the race—if it happens—could create more competition for the kind of small-dollar donations that have so far helped bolster Sanders' run.

The Vermont senator has also faced criticism on the Left for his track record on gun control and immigration reform, topics of contention that could turn into political pressure points as the primary season drags on.

"If more people start to think that his race is crossing over the line from novelty act to a more viable bid, the scrutiny is going to intensify and people, including progressives, will start picking apart his policy stances more and more," said Matthew Dickinson, a political science professor at Middlebury College and presidential-elections expert.

For now, longtime political observers caution that it is too early in the primary season to predict with certainty what the ultimate impact of progressive pushback faced by the Sanders campaign will be.

"Presidential campaigns can be a major opportunity for groups to press their message. That's obviously what's happening with Sanders," said Elaine Kamarck, a senior fellow at the Brookings Institution who was a policy adviser for Al Gore's second presidential bid. "But there are different stages to the race and we're in the early stages right now. The real game starts with the Iowa caucuses, and a lot can happen between now and then."

Sanders is also far from the only 2016 candidate facing pressure from activists. Black Lives Matter protesters are putting pressure on Clinton and O'Malley, and they showed up at a rally for Jeb Bush on Wednesday. Across the political spectrum, 2016 contenders are guaranteed to face a steady stream of demands from supporters and critics alike.

But as the 2016 contest's crowd-favorite progressive champion, Sanders is in a unique spot.

A burst of populist appeal, grassroots organizing, and celebrity endorsements have catapulted Sanders from the relative obscurity of the Senate to the limelight of the campaign trail in a matter of mere months.

That means more visibility—and some degree of validation—for progressive causes. But Sanders's newfound fame is likely to also keep creating headaches for the campaign.

Sanders may be able to strengthen his candidacy by listening and responding to the wide array of progressive demands he faces. But as the campaign reacts, it faces the challenge of knitting together a broad coalition while attempting to prevent it from bursting at the seams.

The fact that the campaign has been fairly responsive so far also sends a signal to other activists that if they push hard enough they may be able to influence Sanders's campaign—a precedent that could open the floodgates to even more pressure.

"Organizing around social justice issues is always challenging," said Winnie Wong, a cofounder of People for Bernie Sanders, a grassroots group supporting the candidate. "Of course there's going to be some trials and tribulations, but I think they're doing a great job of listening and observing what's happening in the grassroots and making their best effort to adopt our positions."