Why did political parties and special interest groups form? what roles do they play in shaping public policy?

Political parties and interest groups often run campaigns completely independently of candidates and their efforts can begin long before and last long after an election season. Both groups act as intermediaries between the public and the officeholders and for many voters are the basis on which the decision of who to vote for is made. In many cases, voters will decide in a cause or a political party to support before they even consider the candidates themselves. The goal line of selecting public officials is what bonds political parties and what differentiates them from other groups. The two key political parties are the democrats and republicans which work in a different way but both endeavor to attain the similar electoral end. An interest group or a group of people with the communal goal of manipulating public policy are different from political parties in that they do not run their individual candidates for office and they classically seek more precise policy goals than parties. The main way they have an effect on public policy is that they supply evidence to the legislators or the political parties.