> I've not seen any evidence that US voters in the US electoral system have much say in the choices they are given on the ballot.
The party nominees are elected by the very same voters—a subset of them, anyway, but anyone can show up if they please—mostly via primaries, sometimes via caucuses.
It varies considerably depending on where you live.
The Republican party in my state decided to do a caucus instead of a primary election for the presidential nominee. This resulted in less than 10% of the registered Republican voters going to the caucus, effectively disenfranchising 90% of the registered Republican voters.
This issue became more evident when the Republican party's state governor candidate chosen at the caucus was not the incumbent governor. The incumbent governor ran a signature petition and was able to get on the primary ballot (which did not include a presidential candidate as described above) and went on to win the primary election and the general election. There were lawsuits and a write-in campaign for the governor candidate who was chosen at the caucus, but that candidate received less than 10% of the votes in the general election.
This illustrates how the state Republican party leadership is out of touch with the state's cititzens and how in the case of the presidential candidate they did not trust that the citizens would do what they wanted in choosing the party's presidential candidate.
The party nominees are elected by the very same voters—a subset of them, anyway, but anyone can show up if they please—mostly via primaries, sometimes via caucuses.