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."