It is hard to say how much of an impact interest groups have on government decisions. This is true because it is hard to know, in any given situation, how a government official would have acted if there had been no interest groups involved in a given debate. We cannot, for example, go back and redo the debate over “Obamacare” and see how it would have turned out if there had been no input from the healthcare lobby.
However, we can still at least speculate about the ways in which interest groups can affect government decisions. There are a number of ways in which they may do so.
First, interest groups can influence which issues become part of the government’s agenda. There are many things that seem to some people to be issues (for example, some people are very worried about the rights of animals), but which do not become part of the government’s agenda. Interest groups can help put issues on the agenda. For example, the issue of gay marriage was not on the agenda to any great extent 15 years ago, but action from gay rights interest groups helped put it there.
Second, interest groups can influence the way legislators vote. I am not saying here that interest groups corrupt legislators with money. Instead, I am saying that it is possible that interest groups can persuade legislators with their arguments and/or intimidate them with evidence that they (the interest groups) represent large and motivated voting blocs.
Finally, interest groups can affect the ways in which laws are carried out. After a law is made, the executive branch has to carry it out. Bureaucratic agencies typically write rules to put the laws into effect. Interest groups with lobbyists can affect the rules that are being written because they can lobby the people who write the rules.
All of these are potential points at which interest groups can affect government decisions. However, it is not clear how much of an impact the interest groups can have on those decision.