How has the nomination of the presidential candidates changed over the years ?

Originally, candidates were chosen at national conventions, however this caused a lot of disputes and near violence in the 1968 election causing politicians to realize this process needed to change. They decided states either had to hold primaries or party caucuses who would chose delegates at the county, congressional district or state level, who would go on to the national convention. Most states opted for the primary option due to its simplicity, 37 of the 50 in 2008 utilized primary elections to select their delegates. National conventions nowadays are really just to present an attractive image and message to citizens watching the action at home on television.

hope this helps