We have a two-stage voting system in the US. The first stage is often a multi-way election. The second stage is usually narrowed to two major candidates, which

We have a two-stage voting system in the US. The first stage is often a multi-way election. The second stage is usually narrowed to two major candidates, which makes the outcome somewhat clearer because one gets a majority, or at least a plurality. But in that first stage you really do have the chance to vote for anybody you like. The incumbents certainly have a thumb on the scale in the primary. That's to be expected: they are already doing the job, and you know that people liked them once. But they can be replaced. It's just a matter of getting enough people to get more votes for your preferred candidate. Getting them to vote for "somebody else" isn't sufficient.