How do special interest groups impact the democratic process?

Special interest groups help to shape public policy by giving money to the campaigns of various candidates.  Some of this money may be in the form of open contributions while other funds may come through super PACs. Candidates who take these funds are thus tied to the special interest group's agenda and will vote favorably on bills that help the interest group.
Another way interest groups help to shape public policy is by attracting blocs of voters to support a candidate. Special interest groups have strong organizational skills and media campaigns that make their issues seem quite clear. The side favored by the interest group is right, while any other view is wrong and must not be supported. In this manner special interest groups can deliver large amounts of support to candidates.
One example of a special interest group is the Anti-Saloon League led by Wayne Wheeler. Wheeler led the fight for Prohibition during the early 1900s. He garnered support from industrialists and church leaders who were against drinking for various reasons. He took up a great deal of money to support candidates who would vote to outlaw alcohol consumption. Even if a candidate liked to drink personally, if he took money or publicity from the Anti-Saloon League, he would have to support Prohibition.  Wayne Wheeler was one of the most influential lobbyists in American history. He was a major factor in getting the Volstead Act passed even though it was not popular in many parts of the country and nearly impossible to enforce. The Anti-Saloon League was one of the most influential special interest groups of its time.