Party identification becomes somewhat formalized when a person runs for partisan office. In most states, this means declaring oneself a candidate for the nomination of a particular party and intent to enter that party's primary election for an office. A party committee may choose to endorse one or another of those who is seeking the nomination, but in the end the choice is up to those who choose to vote in the primary, and it is often difficult to tell who is going to do the voting.
The result is that American political parties have weak central organizations and little central ideology, except by consensus. A party really cannot prevent a person who disagrees with the majority of positions of the party or actively works against the party's aims from claiming party membership, so long as the voters who choose to vote in the primary elections elect that person. Once in office, an elected official may change parties simply by declaring such intent.
At the federal level, each of the two major parties has a national committee (See, Democratic National Committee, Republican National Committee) that acts as the hub for much fund-raising and campaign activities, particularly in presidential campaigns. The exact composition of these committees is different for