Bernie-mentum: Huge 2016 crowds stun observers -- and even Sanders himself
Senate Budget Committee ranking member Senator Bernie Sanders (C), I-Vermont, speaks during a news conference to discuss legislation to restore pension guarantees, in front of the US Capitol in Washington, DC, June 18, 2015 (AFP Photo/Jim Watson)

Bernie Sanders, the rumpled American rebel running for president, has been greeted by swelling numbers of supporters at campaign stops, a development which has stunned observers, especially Sanders himself.

The two-term Senator from rural Vermont hits the stump Saturday in swing-state Colorado, where more than 5,000 people -- believed to be the largest campaign trail crowd for any candidate in 2015 -- have registered to attend, aides said.

"I really was surprised. We've had very, very large and enthusiastic crowds," Sanders told AFP in a brief interview as he emerged from the US Capitol between campaign trips.

"I think we're touching a nerve."

Sanders, an independent seeking the Democratic nomination, has called for a political revolution, warning of America's creep towards oligarchy where candidates are "beholden to the billionaire class."

Democratic frontrunner Hillary Clinton has the name recognition and the infrastructure. Republican Jeb Bush is winning the money war.

But "Bernie-mentum" has seen unprecedented numbers of voters lining up to hear the mantra from Sanders, a self-described Democratic socialist who in the 1980s served as mayor of Burlington, where he nurtured a progressive environment that lives to this day.

He is capitalizing on disillusionment with America's weak economic recovery and an ever-widening rich-poor gap.

More than 800 people turned out for him at a university in Iowa, the state that votes first in the 2016 nominating process.

In Minnesota some 4,000 people heard Sanders rail against income inequality, a message the 73-year-old has been promoting for decades.

"I think the American people are sick and tired of an economics in which 99 percent of all the money is going to the top one percent," he said in the interview.

"They want fundamental change in the way we do economics in this country and they want fundamental change in a political system which allows billionaires to buy elections," he added.

"So I think the message that we're bringing forth is resonating."

Polls suggest he may be on to something.

The Wisconsin Democratic Party convention's straw poll last week put Sanders at 41 percent support, eight points behind Clinton, the candidate long seen as immune to challenges from a thin Democratic field.

In a Suffolk University poll of Democratic New Hampshire voters, Sanders drew 31 percent to Clinton's 41.

"I'm not surprised" about Sanders's rise, Senator Patrick Leahy, a fellow Vermonter, told AFP. "He speaks out to issues."

- From sideshow to contender? -

Whether Sanders can shift from political sideshow to national contender is perhaps the biggest question in a candidacy driven less by corporate and special interests and more by blunt messaging from a largely unscripted grandfather who looks and sounds like he is speaking through a megaphone, even when he's not.

"They think he's credible," Tad Devine, a Sanders advisor and strategist, said Thursday in explaining the crowds.

"Bernie is in... the voter zone," urgently addressing wealth distribution, campaign finance, climate change, and paid worker leave, Devine said.

"Those core issues that he's been talking about are the reason he is resonating so powerfully with people."

Sanders has little campaign infrastructure, and prides himself on having no so-called super PAC, which can raise unlimited funds to support a candidate.

But the money is flowing in, mostly from small donations. Sanders set a goal of raising $40-$50 million before next year's New Hampshire primary in order to be competitive.

"We're on track to raise that much," Devine said.

Despite the poll bump, Bernie's energetic campaign vibe and his willingness to address hot-button issues head on, experts hesitate to place him in the top tier with Clinton.

Ultimately he is an "outsider in what is very much an insider process," according to the latest assessment by the University of Virginia's Center for Politics.

And Bernie is going up against perhaps the ultimate Washington insider: ex-secretary of state Clinton, a former senator and first lady.

"It's going to be a tremendous challenge," Devine conceded.

"There's a story to tell, and as we tell it powerfully, and he performs at a presidential level, we're giving people an alternative to her."

Even if Sanders fades, he has already affected the race by pressuring Clinton, currently neutral on trade, to oppose legislation that fast-tracks a huge accord being negotiated with Pacific nations.

"It is a failed trade policy, and I would hope that the secretary joins... the vast majority of Democrats in the Congress in saying 'no,'" Sanders told CBS Sunday.