Story highlights Hillary Clinton's campaign spent nearly 90% of the money it raised over the last three months

Clinton aides attribute high spending rate to investing in the primary early on

By comparison, Bernie Sander's campaign had a 50% burn rate in the same time period

Boston (CNN) Hillary Clinton's presidential campaign spent 90% of the money it raised over the last three months, campaign aides said Thursday, a staggeringly high figure for a political organization that has prided itself on being thrifty.

Aides announced Wednesday night that Clinton's campaign raised $28 million from July through September, the most a non-incumbent campaign has raised in the third quarter of a non-election year. But on Thursday morning, a Clinton spokesman tweeted that the campaign currently has $32 million in the bank, meaning it spent nearly 90% of what it brought in.

It is no secret that Clinton is running an expensive primary campaign. She has a sizable operation in all early states -- including 78 paid organizers in Iowa and 50 paid staffers in New Hampshire -- and travels with a large number of aides.

She also regularly travels on private planes, an expensive practice that her aides say allows her to do more events and fundraisers in a day.

Clinton aides argued Thursday that their high burn rate is a sign they are investing heavily in the primary. "In addition to television ads, we continued to invest heavily in list building and data with a large analytics and data team," a spokesman said. "We have put a premium on organizing and built a formidable team early."

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