During the 1940s, Tom Parker assembled a team around Eddy Arnold that he would eventually use with Elvis Presley in the 1950s. In order to better understand how Elvis' career progreesed, one might simply look at what Parker did with his first substantial client, The Tennessee Plowboy himself.
This delightful picture from the cover of Billboard in April 1949 shows some of the key players in Presley's story. From the right there's Tom Parker (manager), Jean Aberbach (part owner of Hill & Range music publishers) and Steve Sholes (RCA's head of hillbilly and race records, and Eddy's producer), then Eddy himself.
If one were to replace Eddy Arnold with Elvis Presley, the right side of this picture would fit perfectly into 1956. One of the reasons Parker did so well by Elvis during the 1950s was that he'd already trodden this path before, albeit on a much smaller scale.