The 1952 political conventions were the
first ones covered by TV for a national audience.
Up to that point political party
conventions were controlled by big city bosses from Chicago,
Philadelphia, New York, Boston and a few Southern states. Since 1952
the party bosses and their machines have largely disappeared.
I would suggest that TV has two
qualities that brought about this change. First, TV can not show
political negotiation of the type that bosses and their machines
prior to 1952 carried out, in an entertaining fashion.
That meant
that states that already had political primaries became increasingly
important in conventions and more states opted for primaries.
Second, TV, with its advertising vehicle offered a more effective way for candidates to reach voters than the old publicly embarrassing bribe machines.
So, a technology, TV fed into American's distaste for back room bosses and boss based city machines. We got, in its place, big time billion dollar campaigns and elections that begin two years before election day.