MANASSAS, Virginia (AFP) - At 74, Bernie Sanders is this year's Democratic phenomenon in the White House race, urging Americans to launch a "political revolution" against billionaires and elites - including his chief party rival, Hillary Clinton.

A substantial slice of Americans are frustrated with how Democratic leadership - nearly seven years into Barack Obama's presidency - has bumped up against the hard limits of political power, especially under a Republican-led Congress.

They claim that reducing inequality is a matter of leadership and political will - and the jettisoning of a political class that is beholden to special interests.

Mr Sanders, the independent senator from Vermont, is no newcomer to politics: he served as mayor of Burlington from 1981 to 1989, and has been entrenched in Congress for nearly a quarter century.

But to his supporters, who gathered by the thousands Monday night in a field in Manassas, Virginia, about an hour from Washington, "Bernie" is a new kind of candidate, financed by small donations and not checks from mega-donors, according to several people interviewed at the campaign stop.

"I'm just tired of business as usual, the status quo with politicians. They're all bought by big banks and lobbyists," said 29-year-old student Alissa Rodley, who acknowledged she was attending her first-ever political rally. "Hillary Clinton is very status quo. She is paid for by a lot of the big banks."

Democrats who back Mr Sanders say they don't dislike Mrs Clinton. Some of them even supported her in her 2008 presidential quest.

But after her 20 years in Washington, they doubt Mrs Clinton's loyalty to the cause and criticise her apparent willingness to put deal-making and political modulation above adherence to progressive goals.

While she remains the frontrunner, her poll numbers have slumped amid lingering suspicions about her use of a private email server while secretary of state.

A gay couple arriving to hear the Sanders campaign pitch recalled how it took Mrs Clinton until 2013 to back same-sex marriage.

"It's more that it's in vogue," said Galen Tim, 22, a violinist who was quick to note that Sanders opposed the 1993 "don't ask, don't tell" policy of then-president Bill Clinton banning gays from openly serving in the military.

When Mrs Clinton talks about reaching out to Republicans, fans of Mr Sanders balk, while buying US$20 T-shirts that urge people to "join the political revolution". The villains? Big banks, Wall Street, and their fat-cat patrons.

"We are the 99 per cent, and it's time we take power away from the one per cent!" boomed Mr Sanders, an avowed democratic socialist.

"I am sending a very simple straightforward message to the billionaire class, and that is: You cannot have it all."

His campaign speech is a series of warnings to Wall Street, lobbyists, and the billionaire industrialist Koch brothers, mixed with support for tax hikes on the wealthy, tuition-free college, and paid leave for workers.

For Dave Jennings, 65, the Sanders revolution sounded like a pipe dream at first, but he now takes it seriously.

A longtime Obama fan, the nearly retired government contractor is disappointed that the current administration has not been able to jail many Wall Street executives after the financial crisis.

"Bernie might not be able to go after them either, but at least he's going to try, and make some noise about it," Mr Jennings said.

"I like the idea of wealth redistribution," said Deby Chapman, a Manassas housewife. "He's not afraid to put it out there."

To demonstrate that anything is possible, Mr Sanders is one of the only candidates who dares compare his country to the rest of the world.

He lamented that the United States imprisons more people than China, and deplored that his is the only developed country without broad paid maternity leave.

His pointed critiques have earned praise, but Mrs Clinton too has offered detailed proposals on several issues dear to Democratic Party faithful, including on middle class expansion, family values, immigration and campaign finance reform.

But Mr Sanders supporters assert that the nomination race is about authenticity, not a policy competition.

They also dismiss concerns about age. Mr Sanders is the oldest candidate in the race, Democrat or Republican, and is six years older than Mrs Clinton, who turns 68 in October.

After 80 minutes of stump-speaking and handshaking, Mr Sanders appeared relieved to get back to his car.

Nothing to worry about, insisted Ms Rodley: "He has the vigor of a 25-year old."