Imagine the following scenario: On the 9th of January 2024 in the morning we wake up to news that President of the USA Joe Biden suffered a stroke and died in White House. The administration confirms that the death was entirely natural and no third party contributed to it. Even before the news reach all TV stations another one breaks: Former president of the USA Donald Trump has suffered a cardiac arrest and died in his residence in Mar a Lago. Early reports claim that the site was secure and no malicious actor could reasonably reach Mr. Trump so the death is most likely natural.
What happens then?
Sanely, the vice president takes over for the duration of the term and the primary goes as usual, just missing the two previous top picks. That being said we live in a society with a disturbing amount of violent conspiracy theorists so no telling what the aftermath of that one would be.