Final answer:
The Democratic National Committee and the Republican National Committee keep their respective political parties operating between national conventions.
Step-by-step explanation:
The institution that keeps a political party operating between conventions is the party's national committee, specifically the Democratic National Committee (DNC) and the Republican National Committee (RNC) for the United States' two major parties. These committees, along with their committee chairs, are responsible for maintaining the party's operations, organizing party activities, fundraising, and crafting party strategies until the next national convention. During the conventions, delegates gather to finalize the party platform, choose the presidential and vice-presidential nominees, and set the agenda for the upcoming election. However, between these events, the national committees play a crucial role in keeping the party organized and prepared for the next election cycle.