Democratic presidential candidate Hillary Rodham Clinton announced this week that her campaign committee has raised $45 million. (Greg Allen/Invision/AP)

Seven-figure checks from billionaires Haim Saban and George Soros helped a quartet of independent groups supporting Hillary Rodham Clinton’s 2016 Democratic presidential bid bring in more than $24 million in the first half of the year, officials said Thursday.

Also on Thursday, aides to iconoclastic Clinton challenger Bernie Sanders (I) said the senator from Vermont raised $15 million in largely small-dollar contributions over the past two months.

It’s a strong showing of grass-roots support for an outsider candidate, but Sanders lags far behind Clinton’s reported haul of $45 million for her campaign committee. The numbers underscore Clinton’s status as the clear front-runner in both fundraising and the polls, with Republican hopeful Jeb Bush as her biggest potential rival so far in terms of the presidential money race.

Also this week, retired neurosurgeon Ben Carson announced that he has raised more than $10.5 million since March for his underdog campaign for the GOP nomination. He, Clinton and Sanders are the only 2016 candidates to disclose their early fundraising totals, which are due to be reported to the Federal Election Commission later this month.

Priorities USA Action — a Democratic super PAC poised to serve as Clinton’s top advertising ally — pulled in $15.6 million through June, including $2 million from Saban, a Los Angeles-based media investor, said officials with the group. The longtime Clinton supporter, who held one of the first fundraisers for her 2016 campaign this year, is expected to contribute even more to the super PAC.

Sen. Bernie Sanders (I-Vt.) is drawing big crowds and a large number of contributions from grass-roots donors as he challenges Clinton for the Democratic nomination. (Jim Mone/AP)

More than $12 million of the Priorities USA contributions were received in the past four weeks, after a staff shake-up in which Guy Cecil, a former top Clinton aide, took the helm of the super PAC as its chief strategist. The move sent a strong signal to Democratic donors that Clinton wants them to support the organization, which until then had been struggling to secure big donations.

Among the top contributors to Priorities USA were Soros; DreamWorks Animation chief Jeffrey Katzenberg; financier Donald Sussman; director Steven Spielberg; producer J.J. Abrams and his wife, Katie McGrath; philanthropist Barbara Lee; and the Plumbers and Pipefitters Union, according to a person familiar with the donations who spoke on the condition of anonymity because the final report has not been filed with the Federal Election Commission.

“We have a lot of work to do in the months ahead, but we are starting to see some real momentum,” Cecil said in an e-mail that went out to supporters Thursday.

Meanwhile, American Bridge and Correct the Record — two other super PACs backing Clinton — together brought in $7.7 million. The nonprofit arm of American Bridge raised an additional $1 million.

Soros gave $1 million to American Bridge, which focuses on opposition research, making him the group’s biggest benefactor for the period.

American Bridge founder David Brock said the organization had its most successful six-month fundraising period since its launch in 2011, drawing backing from a mix of longtime supporters and new donors.

The $24 million haul by the pro-Clinton groups is probably a fraction of the amount raised by a super PAC supporting Bush, which is expected to have close to $100 million.

Sanders has 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 about 250,000 people. The average donation was $33.51, the campaign said.

When other sources of revenue, such as sales of T-shirts and bumper stickers, are included, Sanders’s campaign raised nearly 87 percent of its money from donors who gave $250 or less, it said.

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

A poll released Thursday showed him gaining ground on Clinton in Iowa, the state with the nation’s first caucuses.

Among likely Democratic caucus-goers, Clinton leads Sanders 52 percent to 33 percent, according to the Quinnipiac University survey. In a survey from early May by the same pollster, Sanders drew 15 percent support, and Clinton had 60 percent.