Bangladesh, Nov. 11 -- Joe Olson was once such a passionate supporter of the National Rifle Association that he pledged to bequeath several million dollars from his estate to the gun organization upon his death.

But the steady drip of investigations and misspending allegations and a shakeup at the top ranks of the NRA compelled him to alter his will. The NRA will no longer get his money.

"The rot had gotten worse and I simply decided: No, I'm not giving those people my money," Olson said.

Olson reflects what has become a new challenge for the NRA as its legal and financial issues stack up: the loss of big donors.

The NRA attributes much of its success and power to rank-and-file members who contribute a few dollars here and there throu...