In 1940s Mass Communication was a nascent field and prevalent paradigm was that of powerful effects of media, that media has direct powerful effects on shaping perceptions. This was supported by an event in 1938 when a Radio Station aired an episode of an alien invasion on Halloween in New Jersey and many people actually believed it causing a nation wide panic
https://en.m.wikipedia.org/wiki/The_War_of_the_Worlds_(1938_radio_drama)
But a study was conducted for the 1948 (I think) Presidential elections by Paul Lazarsfeld to guage people's voting preferences based on media exposure. It found that there were four categories of voters.
Early deciders, who made up their mind and voted accordingly regardless how media presented stuff. Waivers, people who thought of changing vote after watching media but eventually voted for their intial choice. Converts, as implied who changed vote after media exposure and crystalizers, people who just made up their mind on the voting day who to vote for based on media exposure.
Study found Early deciders and Waivers formed the majority and concluded media doesn't have powerful effects but rather limited effects on shaping perceptions.
The study also gave the idea of two-step flow of communication. It found that people were more influenced by how people around them interpreted media content rather than direct effect of media i.e Famous YouTuber doing his reaction of Trump Kamla debate might influence you more than watching debate yourself or even your dad or uncle giving his analysis might influence you more
This study was monumental as it brought a new paradigm in the world of communication studies and gave impetus to further studies that shape our discussions till today.