Their purpose, internal politics, and methods for accomplishing their goals are all varied. To begin with, the purpose of a political party is to gain power by winning elections by supporting candidates, and to help them win by advertising and fundraising. Unlike political parties, the purpose of interest groups is not to elect members into public office, but instead have a much narrower objective and seek to promote a position on a specific issue, such as gun control or agriculture.
For political parties, internal politics is much more flexible than it is within interest groups. Single issues, such as gay marriage, may not be agreed upon by members, but they generally agree on the majority of issues.…