Are interest groups a danger to democracy?

Special interest groups buy votes by making "legal" (READ: Questionable at best) donations to Congressional Representatives and Senators. The problem is there are few rules to keep our elected leaders from taking these badly-shrouded bribes, and the rules they have are not enforced. Also, bear in mind many people in Congress own stock in oil companies and corporations that are big in tobacco like Kraft-Philip Morris, so they have a vested interest in not restricting their profits.