The Voter Decides is a 1954 political science book by Angus Campbell, Gerald Gurin, and Warren E. Miller. It first developed the notion of "party identification" or, in abbreviated form, "Party ID." Party ID is the sense of personal attachment the individual feels toward the political group or party of his or her choice.
The Voter Decides developed three theories as to how a person acquires Party ID.
In short, once a Party ID is acquired (through unknowing absorption from a very young age) the person spends the rest of his or her life adopting views and positions that create a reason for their party affiliation.