Voting is a core principle of any functioning democracy. While the American system is a Democratic Republic, voting is still instrumental in assigning authority figures to lead the country. Anytime we go to cast a vote, or even if we don't, we decide on that item that is up for a vote. That decision must become informed by something. If you don't vote, you've decided that the thing up for a vote isn't significant enough to engage your participation in making the decision. If you do show up to vote, though, then you are going in to mark that ballot with a set of reasons that have motivated you to vote in the manner you do. What determines why people vote the way they do? While a voter may have contemplated many pros and cons of electing one person over another, their decision will always get determined by one issue. Therefore, I believe that all voters are single-issue voters in the end.
You may be thinking, especially if you are politically engaged, that there is no way I am making my decision just off one factor. You may feel that you weigh all the policy debates and make the best decision based on the sum of the pros versus the sum of the cons. Even then, your final decision gets based on one issue that you find pertinent. We should first note that many voters are not this conscientious about the contributing factors of their vote. For those who are making a more conscious effort to review policy plans, follow debates, and understand the positions each candidate is standing for, they may realize and clarify why they voted the way they did. Yet, whichever type of voter you are, in the end, you are making that decision to cast your ballot with one deciding factor guiding the way.
You may still feel an aversion to this idea. You may be thinking, how could I be a conscientious voter who thought long and hard about the decision I made and still be considered a one-issue voter. It may not feel like you are relying on just one issue to decide; the options on the ballot determine how easy it is to notice, though. You'll more than likely know who you are going to vote for after a quick comparison. Now let's say you are undecided, and you are examining each candidate. You will get to a point where you say this one candidate's policy plan regarding a specific topic is the thing that tips the scales towards you voting for that candidate. This effect of allowing one issue to sway your vote occurs whether you realize it or not or whether you take the vote seriously or not. For example, even if a voter decides on choosing a candidate for nothing other than their name, they are still a single-issue voter. In this case, the issue was their name. As much as I would hope someone plans to vote a certain way for more than just a name, I know it happens. Regardless of the issue, that vote and all other votes get decided by one issue.