The Role of Convention
The first method the parties developed to nominate their presidential candidates was the congressional caucus. That method was regularly used in the election of 1800 to 1824. However, the closed, nonrepresentative character of this system led to the downfall in the mid-1820's. For the election of 1832, both major parties turned to the national convention as their nominating device, and it has continued to serve them ever since.
The Apportionment of Delegates
With the date and the location set, the national committee issues its "call" for the convention. That formal announcement names the time and place. It also tells the party's organization in each State how many delegates it may send to the national gathering.
By tradition, both parties give each State party a certain number of delegates based on that State's electoral vote. Over the past several conventions, bouth parties have developed complicated formulas that also award bonus delegates to thore States that have supported the party's candidates in secent elections.