Story highlights Sheldon Adelson has grown frustrated with his candidate's performance on the campaign trail

"I hate to admit it, but Hillary won it," said John Harris, a California agriculture titan

Washington (CNN) Donald Trump in September spent more than twice what he had in any month prior, parting with $70 million just before his campaign hit its rockiest patch yet, which has unnerved a significant amount of donors, including Sheldon Adelson.

As Trump's presidential fortunes dip lower than ever before, Republican donors are struggling to decide how to react to a candidate who is endangering the GOP's brand and down-ballot prospects with less than three weeks to go.

There was no immediate donor backlash on Thursday in the aftermath of Trump's final debate performance, though many of the Trump donors at this point in the race remain diehards who have weathered months of scandal and controversy and are unlikely to abandon him given the latest one.

Yet the campaign had been spending as if it still has a committed donor base. The $70 million in spending -- $40 million more than what Trump spent in the month of August -- came as he drastically increased his television campaign and as the race tightened.

During that September high point, he boosted his staff to about 170 people, according to payroll records, and spent an enormous amount of his money, over $20 million, on a digital-adveritising firm that does his online fundraising. He also spent about $5 million on a data firm, Cambridge Analytica, which is reportedly owned by Robert Mercer, a Republican megadonor whose family is deeply intertwined within the Trump campaign architecture. He spent $250,000 on that same firm a month prior.

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