Political parties are distinguished from interest groups in American politics by the fact that interest groups do not:

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Once an interest group sponsors a candidate for political office under the banner of their organization, they become, by definition, a political party. Otherwise, interest groups can behave very much like political parties. Like political parties, they can clearly support candidates through both finances and mobilization. Like political parties, interest can organize various politicians to work together toward common goals.

Political Parties Are Distinguished from Interest Groups in American Politics