U.S. Treasury Secretary Timothy Geithner said on Wednesday that Americans can't let fear of future deficits stymie necessary government investment in projects that are needed to spur growth.

Speaking to the Chicago Economic Club, Geithner tackled Republican opponents of the Obama administration who claim that spending is excessive.

"There is no economic or financial case for using the fear of future deficits to cut as deeply into core functions of the government, to weaken the safety net or fundamentally alter Medicare benefits as do the Republican proposals," Geithner said.

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