Normally I'd agree with you that we shouldn't have a tag that's a perfect subset of another tag. We certainly could replace it with a combination of president and election. However, I think we should make an exception here because of how common questions about (usually US) Presidential Elections are.
There are currently 566 questions in presidential-election, and it's the 8th most common tag. That alone should make us hesitant to remove it. Beyond that, it shows that presidential-election is a distinct topic that people are actually using to identify their questions. That makes sense: presidential elections are a big deal, especially in the US which is home to a disproportionate amount of our users, and people come here with questions about these elections, not about elections or presidents in general.
presidential-election provides a clear way to organize and find the many, many questions about US (or French, etc.) presidential elections and I think the tag should remain