People who lobbied the federal government in 2007
"The most concrete number I can offer you *15,103*. That is the number > of people who lobbied the federal government in 2007. This is a > low-end estimate of the number of lobbyists in DC. There are > undoubtedly many more who don't register but still influence policy, > such as PR executives, grassroots organizers and others. There are > also lobbyists who did not lobby in the past year but may still be > active. Since there's no way to accurately count these groups, we > stick with the number who lobbied in a particular year."
Source: Greg Gasiewski, Lobbying Researcher, Center for Responsive Politics, 19 March 2008.