For the most part, the general public participates in choosing the presidential nominee. You aren’t allowed to vote multiple times, you have to use your vote within a party when you go to the booth (you have to choose, say, Dems or GOP, but depending on who is standing, you could choose Greens or Libertarians or Constitutionalists or… But these are incredibly small and rare). If you want to vote for an independent, then it is a bit different. You sign a petition attesting that your candidate has the support to contest in the general election. If you sign a petition, you cannot vote in party primary, and vice versa.
Given this reality, American general election voters between the nominees of different parties have no right to complain, in my view, about if a party nominee is unappealing or status quo. Bernie Sanders ran inside (and is running inside again) the Democratic Party, and came in close second to Hillary Clinton. Trump has jumped primaries as a voter his entire life, voting for anyone, a pretty good example of an independent, but by choosing to run in the Republican primary, winning the nomination, and then winning the presidency, he has become the leader of the GOP. Typically, the president is always the leader of his (so far always his) party, but if the White House isn’t held, then the party in opposition’s leader is the National Chair. For my party, that is currently Tom Perez. Under Obama, the GOP’s head was Reince Priebus, who became Trump’s first chief of staff. The National Chair is chosen by party leaders, but presidential nominees are chosen by voters. President is chosen by voters but then their votes are represented by the electors of the electoral college. Which, due to the congressional split I mentioned earlier, is causing problems. Democrats have to secure both the popular vote and the electoral college to win. GOP nominees only need to secure the electoral college because of voter demographics. Unfortunately. This is why there is the Popular Vote Compact which is a coalition of states promising to assign their elector votes to the winner of the popular vote. Most of those compact states are solidly Democratic, however, which won’t change a damn thing. We need the EC to be reformed or removed, and that requires a Constitutional Amendment, which requires an act of 2/3rds of Congress (again, that means Senate has to help) or a Constitutional Convention, and ratification of 3/4ths of the States and… Yeah. Not gonna happen.