I think that the kind of people who were still big Elvis fans by the 1970s would accept that it was an accident, and not bark too much for his blood. For some reason, I have the idea that there was probably a lot of overlap between Nixon supporters and late-career Elvis fans, and the people in that Venn overlap would not be inclined to think the worst of the King.
Now, if JOHN LENNON had accidently shot Nixon, it would be a very different story. Though it's pretty implausible that he'd be having a friendly meeting with Nixon, much less one where he was allowed to bring a loaded gun.