Early in his candidacy, Herman Cain may have accepted tens of thousands of dollars in goods and services for his campaign from a tax-exempt organization founded by his top aide, documents from the organization show, raising the prospect of serious violations of tax and election law by both Mr. Cain’s campaign and the organization.The documents suggest that the nonprofit organization, Prosperity USA, effectively subsidized some early costs of his presidential bid, paying for computer equipment, charter planes and air travel for Mr. Cain or the aide, Mark Block, who is his chief of staff.Such expenditures would violate federal election and...