> Coming from a UK background something I've been long curious about is is there a constitutional reason for when the opposition presidential candidate is selected.

That's a very interesting point. On the other hand, the GOP did have a leader through the Biden administration - Trump.

Even when they don't, such as under Obama, they do have effective means (Fox, social media, etc.) and content (effective, disciplined talking points) of communication. The Dems have neither.