The structure of the electoral process is such that voters first choose among potential nominees for the parties; this is the primary process.
After the primary elections, we have the general election. In the United States, two parties are dominant. They are called the Republicans and the Democrats.
In most presidential elections, including this one, the overwhelming likelihood is that the winner of the general election will be either a Republican or a Democrat. Unnominated Ideal Candidate Pulling the Lever For Whom Will Erase All Operative Political Realities is not an option on the ballot.
Preferring one candidate to the other listed on the general election ballot does not preclude any possibility of affecting political life for the better over the next four years. Nor does preferring one candidate to the other constitute the voter’s affirmation that he endorses all policies and embraces all flaws of the candidate that he prefers.
What preferring and voting for one major-party candidate rather the other does mean is that, given a choice between Candidate A and Candidate B, and regarding the difference between A and B as significant enough to justify making a choice, the voter prefers either Candidate A to Candidate B or Candidate B to Candidate A.