Prior to 1968, the presidential nominating conventions were attended by largely hand-picked delegates. They used committees, proxy votes, delegate instruction, and high participation fees to control the nomination in proverbial smoky backrooms regardless of popular will.
Hubert Humphrey was the last such candidate -- he received the 1968 nomination despite having won NO primaries or caucuses.
Effortsbegan in 1968 to reduce the influence of party leaders over delegate/nominee selection.