WASHINGTON -- Hillary Clinton’s presidential campaign raised more than that of Sen. Bernie Sanders (I-Vt.) during the third quarter of 2015. But not by much. And with Sanders building a massive pool of grassroots donors, it seems entirely possible that he could end up surpassing her in quarters to come.

All told, Clinton, the former secretary of state, raised $28 million between July and September of this year, a haul that her campaign called the “largest off-year third quarter for a non-incumbent.” That figure falls short of the $47.5 million she raised in the preceding three months, though it still puts her on track to reach her campaign's goal of $100 million goal for the year.

Sanders was right behind her. According to a campaign aide, the senator is on track to raise over $25 million during the third quarter. Early reports put that figure at $24 million, but the aide explained that the campaign was experiencing its best fundraising day of the cycle on Wednesday, helping to stuff its coffers even more.

The haul was far larger than the one Sanders had in the previous quarter, when he raised $15.2 million. And perhaps even more impressive for the senator is the way he's grown his treasure chest. The campaign has received 1.3 million donations from 650,000 donors, which, the Sanders aide noted, is “something another insurgent candidate didn't achieve until after [the] Iowa caucuses back in 2008” -- the candidate being then-Sen. Barack Obama (D-Ill.).

Sanders, in short, isn’t having to work for the money he’s raising. Much of it is pouring in online. Clinton, by contrast, has labored hard for her cash. Though her campaign said that 93 percent of the third-quarter donations were in amounts of $100 or less, she also attended nearly 60 fundraisers this quarter. Sanders, meanwhile, held only seven fundraisers in the same period.

Clinton's $28 million haul was very labor intensive. 58 personally headlined fundraisers, over a dozen in the last week. — Dan Merica (@danmericaCNN) October 1, 2015

Both candidates' campaigns said that the money they raised was for primary funds exclusively. The Sanders campaign revealed it has $25 million cash on hand. On Thursday Clinton's team had about $29 million in cash-on-hand after the second quarter.

UPDATE: 10/1 -- A massive flow of donations on Wednesday helped push Sanders' quarterly total to "about $26 million," according to the campaign.

Just how big was the day's haul? As of midnight on Wednesday, Sanders had received $2.07 million in donations, all online -- a record for the campaign. In the last two and a half hours of the day, the total raised was $502,000.

At about noon EST on Thursday, Josh Schwerin, a spokesperson on Clinton's campaign team, tweeted that the campaign had over $32 million in cash-on-hand and counting.

We’re still counting, but @HillaryClinton has more than $32m on hand after raising a non-incumbent record $28m+ in Q3 — Josh Schwerin (@JoshSchwerin) October 1, 2015