It's not the FBI's job -- it's the FEC's, and they're totally inept. He's taking advantage of every loophole he can find.
Barone has it exactly right: Thugocracy.
Obama also put a hold on two nominations of FEC Commissioners so they are operating without the manpower and when they asked the DOJ to look into this they were told the DOJ doesn’t want to get involved in an election and will look at it after November 4th.
What does that tell you about this Administration?
According to the FBI website, the FBI does investigate election fraud including illegal contributions: