Explain the relationship between interest groups and legislation

Both political parties and interest groups are formally organized.

interest groups are an organization of people who share a common interest and work together to protect/ promote that interest by influencing the government.
Interest groups very greatly in size, aims, and tactics. And as far as political parties use campaign finance committees to raise funds for election purposes.

An interest group is a group of people who come together and organize themselves because they have common interest. They act together to promote it and defend it. Examples of interest groups: trade unions, employers' organizations, companies, professional associations and NGOs.

The  relationship between interest groups and legislation is that interest groups put pressure on elected candidates to promote legislation that favors their causes.

How do they do that? Interest groups send representatives to state capitals and to Washington, D.C. to pressure members of Congress and other politicians. They are involved in a process to influence legislation and politics. This process is known as "Lobbying." It can take many forms and use different strategies. In the United States, interest groups can testify at Congressional Hearings. Lobbyists often contact government officials directly or informally, and present them with research results. They also communicate their findings through different media and cases are known in which they are also involved in supporting the drafting of bills. They also actively participate in political campaigns.

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