At one point in history, individual voters had no ability whatsoever to choose a presidential candidate from even their own political party. Officers and certain influential political party members literally met behind closed doors to choose a presidential candidate in secret. Only when a suitable candidate had been selected would the announcement be made public. Needless to say, this overuse of the smoke-filled backroom technique eventually served to make individual party members feel completely disenfranchised from the presidential candidate selection process.

There is no provision in the original Constitution which calls for primary elections or caucuses. When the Constitution was written, presidential elections were determined by popular vote, with the most popular candidate becoming president and the second largest vote-getter becoming vice-president, regardless of political party affiliation. The idea of holding a partisan primary election to choose a presidential candidate is largely a 20th century invention.

If you participated in a primary or caucus, you may have noticed how partisan it was. Both major political parties hold their own primaries in each state to allow registered party members to vote for their preferred candidate. What voters are actually voting for is a percentage of delegates who are bound to vote for their candidate at the national party's convention. Each state has a designated number of delegates according to population. There are also a few well-connected party officials who attend as super-delegates, meaning they are not bound to any one candidate.

Let's assume the fictional state of West Dakota had 100 Democratic delegates based on its size. Democratic candidate A might receive 55% of the total primary votes, while candidate B receives 25% and candidate C gets the remaining 20%. Under the Democratic party's system, this would translate to 55 delegates pledged to Candidate A, 25 pledged to Candidate B and 20 pledged to Candidate C. When the national Democratic convention is held, these delegates are obligated to vote for their designated candidate during the first round of nominations. If a candidate does not automatically receive a majority of the votes after the first round, the delegates are free to choose a presidential candidate based on their own personal or political preferences.

The Republican primary in West Dakota works a little differently. If Republican Candidate A receives the majority of the primary votes, he is generally awarded ALL of the state's delegates. Once a particular candidate has earned one vote past a majority of the state's delegates, it often becomes a formality for the national party to choose a presidential candidate. This is why some states have made efforts to change their primary voting dates to an earlier time in order for their delegates to have more of an impact on the overall presidential campaign.

Essentially, both parties officially choose a presidential candidate during their national conventions. Primaries and caucuses allow party leaders to get a sense of the electability of certain candidates, but none of those partisan votes apply towards the mandated general election held in November. Individual voters may express their support for specific candidates during a primary election, but both major parties are free to use their own criteria to choose a presidential candidate at the convention. They generally do follow the will of the majority of their party members, but there is nothing in the rules which would prevent party leaders from selecting an alternative candidate.