The biggest obstacle to Hillary Rodham Clinton winning the Democratic presidential nomination is a rumpled, white-haired grandfather who doesn't even call himself a member of the Democratic Party.

Sen. Bernie Sanders has no entourage or bevy of political advisers. He represents a state with half the population of Hillsborough County, and he has long been viewed by the national media as a quaint fringie — a self-described democratic socialist, of all things! — from the People's Republic of Vermont.

But contrary to conventional wisdom about the Democratic presidential contest, people are listening to presidential candidate Sanders. A lot of people.

"This is a rigged economy and, brothers and sisters, together we are going to change that," Sanders, 73, told a crowd of more than 10,000 people in Madison, Wis., Wednesday night.

"This campaign is sending a message loud and clear to the billionaire class. And that is: You can't get tax breaks when children in America go hungry. You cannot continue to send our jobs to China when millions of Americans are desperately looking for work. You can't hide your profits in the Cayman Islands and in other tax havens when we have so many unmet needs in America!"

The packed arena roared. "Bernie! Bernie! Bernie!"

It was the largest campaign crowd any of the nearly two dozen 2016 presidential candidates has drawn so far, and it followed Sanders pulling 5,500 in Denver the previous week. Three hundred people showed up in late May when he appeared in the farm town of Kensett, Iowa, population 260.

Whether you call it old-fashioned class warfare or, as some Sanders fans prefer, a movement for economic justice, Sanders is tapping into something, much as the less-focused Occupy Wall Street movement did. How broad that something is remains to be seen, but Vermont's junior senator is no longer a bit player in a Democratic primary where Clinton had long been seen as the most commanding frontrunner in generations.

The latest poll of New Hampshire primary voters, by WMUR and CNN, found that Clinton's 21-point lead over Sanders two months ago has dropped to 8 points, 43 percent support to 35 percent. That is within the poll's margin of error. None of the other announced Democratic candidates — former Maryland Gov. Martin O'Malley and former Sens. Lincoln Chafee of Rhode Island and James Webb of Virginia — topped 2 percent.

A Quinnipiac University Iowa poll released last week found Clinton with an overwhelming 19-point lead over Sanders. But the Vermonter's support had doubled in less than two months, and Clinton's lead had been cut by more than half.

"He is galvanizing support unlike any candidate I have ever seen, and I've been doing this for a while," said Mike Fox, a St. Petersburg resident and national organizer for the political group Progressive Democrats of America, which is independently helping Sanders.

The Sanders campaign has no presence in Florida, concentrating instead on the early primary states like Iowa and New Hampshire and generating national attention, buzz and enthusiasm with appearances in liberal bastions like Austin and Madison.

Grass roots Sanders supporters are still mobilizing across the Sunshine State.

In Tampa Bay, nearly 80 people attended a Sanders event June 22 in Tampa, while former Pinellas Democratic chairman and St. Petersburg mayoral candidate Ed Helm last weekend hosted about 50 people at his home. They saw a Skype presentation from one of Sanders' aides in Vermont.

"He's like a breath of fresh air," said Jim Jackson, a Democratic activist in Pinellas. "He's standing up and taking on the issues that so many of us care about: the big banks, the hedge fund people, how nobody went to jail for what they did to the economy, while the middle class keeps getting stepped on."

Like Jackson, retired Tampa lawyer Rochelle Reback said she would support Clinton if she wins the nomination, but so far sees little to be excited about with the frontrunner.

"Bernie Sanders is the one speaking up for the middle class in America and he's brave enough to take positions that are not favorable to Wall Street. It's about time somebody did that," she said. "Hillary has been overly cautious, and I don't think she's been very clear on a lot of her positions."

Helm, who splits his time between St. Petersburg and Vermont, said it's not uncommon to see barns painted with the slogan "Take Back Vermont" — a backlash against liberal Vermonters — and pro-Bernie Sanders signs on the same property.

"He connects with people. People respect his honesty, his directness and his commitment," said Helm, who thinks Sanders' populism and independence would pull tea party Republicans and libertarians in a general election. "He cares about things that matter to everyday people."

Voters do not register by party in Vermont, but Sanders calls himself a democratic socialist. That's not as radical is it may sound. Sanders, who caucuses with the Democrats in the Senate, essentially argues that the government in a healthy democracy can and should play a big role in promoting and protecting the welfare of citizens. His platform includes:

• Committing $1 trillion to rebuild the country's infrastructure and generate jobs.

• Creating a Medicare-for-all, single-payer health care system.

• Raising the minimum wage to $15 an hour.

• Making tuition free for public colleges and universities.

• Aggressively moving the country's energy policies away from fossil fuels.

• Curbing the power and influence of Wall Street and breaking up the biggest banks. ("If a bank is too big to fail, it is too big to exist.")

"The issue of wealth and income inequality, to my mind, is the great moral issue of our time, is the great economic issue of our time, and it is the great political issue of our time," Sanders said in Wisconsin in his Brooklyn baritone. "Let me be as clear as I can be: There is something profoundly wrong when today the top one-tenth of 1 percent own almost as much wealth as the bottom 90 percent. There is something profoundly wrong when today 58 percent of all new income is going to the top 1 percent."

Sanders served as mayor of Burlington, Vt., for four terms, in the U.S. House for 16 years, and eight so far in the U.S. Senate. He may be among the most liberal members of Congress, but he also understands governing and compromise. He is the ranking Democrat on the Senate Budget Committee and chaired the Veterans Affairs Committee from 2013 to 2015.

"You won't find two members of Congress more at the opposite ends of the ideological spectrum than Bernie and me," said North Florida U.S. Rep. Jeff Miller, the House Veterans Affairs chairman. "But from working with him on VA issues, I can tell you Bernie is a very intelligent man, a skilled negotiator and an astute political tactician. I think he has been underestimated and will surprise a lot of people."

Consistently drawing monster crowds does not necessarily translate to winning presidential nominations, as Democrat Howard Dean and Republican Ron Paul can attest. He has shown little momentum among minority voters — which would make him a much more serious threat to Clinton — and he faces the widespread perception that he can't win.

But even as Sanders lambastes the role of big money in politics, it appears money will not be his chief obstacle. He reported last week raising $15 million from 250,000 donors since entering the race in April, with an average contribution of $33.51. Clinton's campaign said she raised about $45 million.

"Right now the people who favor Bernie, we're not fighting Republicans. We're fighting the media's bias in not taking him seriously as a legitimate challenger," said Reback of Tampa. "People go, 'Bernie can't win,' but it's just you guys (in the media) talking to each other. You're not talking to the 10,000 people who showed up in Wisconsin."

In Miami, 28-year-old Ellen Wall clicked on the New York Times website Thursday morning to read about Sanders' Madison event. She found a report on Jeb Bush's strong opposition to peas in guacamole, but nothing on Sanders.

"Yes, Bernie Sanders is a dark horse," she said. "But if more people are able to hear about his message, absolutely he can win the nomination."

Contact Adam Smith at asmith@tampabay.com.