What process involved party politicians and voters gathering to nominate candidates for office?

The party nominating convention is the correct answer because it was a significant political innovation in the early 19th century that allowed for greater participation by party members in the selection of their candidates. These conventions brought together delegates from various local party groups who gathered to formally nominate candidates for political office. This process encouraged broader public involvement and helped to replace the less inclusive methods of candidate selection, such as the King Caucus, where a small number of party leaders made decisions behind closed doors.

The party nominating convention also enabled the mobilization of grassroots support and increased transparency in the candidate selection process. Delegates debated issues, outlined party platforms, and engaged with the party base, thus allowing for a more democratic approach to the nomination of candidates compared to earlier systems. This shift reflected the broader democratic ideals emerging in the United States during this period, making the process more accessible to voters and aligning with the increasing emphasis on popular sovereignty.