The invisible primary is a product of the presidential nominating reform that was instituted in 1972. The reform grew out of the bitter 1968 Democratic nominating race, which was fought against the backdrop of the Vietnam War. The anti-war challenges of Eugene McCarthy and Robert Kennedy drove President Lyndon Johnson from the race. Yet, party leaders, who controlled most of the convention delegates, picked Vice President Hubert Humphrey as the presidential nominee even though he had not entered a single primary. Insurgent Democrats were outraged, and after Humphrey narrowly lost the general election, they engineered a change in the nominating process. State parties were instructed to choose their convention delegates through either a primary election or a caucus open to all registered party voters.
The reform had obvious appeal. What could be more democratic than giving control of presidential nominations to the voters? Reformers did not foresee the extent to which the new system would be brokered by the news media and failed to account for journalists’ limitations as a political intermediary. They are not in the business of sifting out candidates on the basis of their competency and platforms. They are in the business of finding good stories. Donald Trump was the mother lode. During the invisible primary, the press gave him what every candidate seeks — reams of coverage. In his case, even the media’s attacks were a boon. Many Republicans dislike the press enough that its attacks on one of their own are nearly a seal of approval. (Source)