Sen. Bernie Sanders (I-Vt.) on Capitol Hill Thursday announcing his presidential campaign. (REUTERS/Jonathan Ernst)

Advisers to Bernie Sanders have argued that his grassroots network of small-dollar donors could raise him the roughly $50 million the independent senator from Vermont will need to run a credible, competitive campaign in the Democratic presidential primaries.

They may be right.

On Friday, the Sanders campaign announced that it has raised more than $1.5 million online in the 24 hours since he announced his candidacy. It is a surprisingly heavy haul for a candidate whom some in the Democratic chattering class have cast off as a gadfly and viewed as unable to wrest the nomination from the overwhelming favorite, Hillary Rodham Clinton.

The donations came from a broad base of supporters -- some 35,000 donors who gave an average of $43.54 a piece, according to the Sanders campaign. The campaign also said it signed up more than 100,000 supporters through its website, building what it calls a "mass movement."

Clinton has not released any details about her fundraising totals, online or otherwise. But the Sanders haul outpaces the three major Republican candidates who already have announced. In the first 24 hours since launching their campaigns, Sen. Marco Rubio (Fla.) raised $1.25 million and Sens. Rand Paul (Ky.) and Ted Cruz (Texas) raised about $1 million each, according to their campaigns.

“This is a remarkable start for Bernie's campaign," Sanders adviser Tad Devine said in a statement. "People across America are yearning for authentic leadership that tells them the truth about what is holding back our nation."

As a self-described socialist, Sanders does not have a deep base of wealthy donors and corporate PACs who give to his campaigns; he jokes that he doesn't know any millionaires or billionaires. He said Thursday he is taking on the "billionaire class" he says are controlling the U.S. political system, and would make the surge in political spending in recent years -- especially through super PACs fueled by unlimited donations -- a major theme of his campaign.

During a news conference on April 30, 2015, Sen. Bernie Sanders (I-Vt.) spoke about his agenda for the U.S. He also answered questions about how he would campaign against Democratic presidential candidate Hillary Clinton. (AP)