Democratic presidential candidate Bernie Sanders speaks during a political rally on Wednesday in Madison, Wis. (Michael P. King/Wisconsin State Journal via AP)

DES MOINES — Sen. Bernie Sanders (I-Vt.), the iconoclastic Democratic presidential challenger who is attracting large crowds of progressives, raised $15 million in his first two months as a candidate, his campaign said Friday.

The haul marks a strong performance for the underdog candidate, but still puts him well behind Democratic front-runner Hillary Rodham Clinton, who told supporters this week she raised $45 million since mid-April. Other pro-Clinton groups said Thursday that they had raised an additional $24 million as well.

However, Sanders's take is expected to be stronger than other Democratic contenders, including former Maryland governor Martin O’Malley.

O’Malley had not released totals as of Thursday, and neither had Lincoln Chafee, the former governor of Rhode Island, who is also running.

Jim Webb, a former senator from Virginia, joined the Democratic race on Thursday.

Sanders raised his money almost completely online, holding only a handful of traditional fundraising events, including two in Los Angeles, aides said. On the campaign trail, Sanders rails against the political influence of the “billionaire class” and has said he is proud not to have a super PAC supporting his efforts, as most candidates from both parties do.

His campaign said he received nearly 400,000 contributions from roughly 250,000 people. The average donation was $33.51, the campaign said.

When including other sources of campaign revenue, such as T-shirt and bumper sticker sales, the campaign raised nearly 87 percent of its money from donors who gave $250 or less, aides to Sanders said.

The announced fundraising haul comes a day after Sanders had his biggest rally yet in Madison, Wis., drawing more than 10,000 people to a Wednesday night event at Veterans Memorial Coliseum.

A new poll released Thursday showed him gaining ground on Clinton in Iowa, the first-in-the-nation caucus state.

Among likely Democratic caucus-goers, Clinton leads Sanders, 52 percent to 33 percent, according to the Quinnipiac poll. In a survey from early May by the same pollster, Sanders drew just 15 percent support and Clinton was eight percentage points higher.

Sanders is set to return to Iowa on Thursday afternoon for three days of campaign events.