The filings showed Gillibrand, a Democrat, had $2.58 million remaining in her campaign war chest as of Sept. 30, down from the $10.5 million at the end of June.
In total, Gillibrand has taken in $14.6 million during this election cycle, the filings show.
She gained $1.39 million in new contributions in the July-September quarter, according to the filing. By comparison, from April to June, she raised $712,771, and spent only $110,388.
Long, a Manhattan lawyer, reported $96,411 in cash at the end of June, after spending $236,564 and taking in $319,976 in contributions since January. Her husband, Arthur Long, also a lawyer, was among her largest contributors, with two $2,500 donations in March, campaign records show. With $17,000 in reported loans to the campaign, her total receipts as of the end of June were $336,976.
A campaign spokesman for Long didn't immediately respond to a request for her latest disclosure report or for comment.
Gillibrand's office has run three ads to date in her contest with Long -- ads that her spokesman Glen Caplin described as "all positive." He noted that the campaign has reserved considerable airtime through Election Day. Long's ads have portrayed Gillibrand as the backer of an "Obamacare" policy that has cost jobs.
Gillibrand was leading Long by a 64 percent to 27 percent margin among likely New York voters, according to a Sept. 13 Quinnipiac University poll. "Republicans back Long 71-23 percent, but Gillibrand takes every other group by wide margins," Quinnipiac said.