The Center for Responsive Politics said in a report -- "Lobbyists 2012: Out of the Game or Under the Radar?" -- that it tracked former registered lobbyists. It said 46 percent of lobbyists registered in 2011 but not in 2012 were working for the same organizations.
The center said the number of lobbyists has been falling since 2007 and spending dropped in 2011 and 2012.
The sour economy and other factors are responsible for some of the decline, CRP said, along with the 2007 Honest Leadership and Open Government Act and an Obama administration rule limiting hiring of former lobbyists.
But the law allows some former lobbyists to continue to exert influence without registration, the group said.
"By working as policy advisers and in other 'non-lobbyist' positions, former lobbyists can keep their current jobs but escape the consequences of being a registered lobbyist, leading people in and out of lobbying to suggest that those consequences act as a deterrent to transparency," the report said.
The group cited former House Speaker Newt Gingrich, R-Ga., and his consulting company, the Gingrich Group. Freddie Mac, the Federal Home Loan Mortgage Corp., paid the group $1.6 million in fees from 2001-10.
"News reports say Gingrich met with members of Congress regarding issues salient to his clients and yet he never had to register as an official lobbyist," the report said.