What is one major role of political parties in U. S. politics

One major role of political parties in U. S. politics is to nominate candidates for political office (such as The President, Vice President, Senators, Representatives, etc.) that represents what the party stands for; likewise, another major role is to communicate their philosophy to people, make campaigns and a series of activities in order to get their candidates elected, because once their candidates are in office, the party will have more possibilities to make legislation and program initiatives that are in tune with their philosophy on how to lead the nation.