My guess is the republicans themselves or atleast someone right leaning.The question is by who?
Biden does not stand to benefit from Trump surviving an assassination attempt.
And if he did order this, I’m sure he would be able to get someone better than that.
My guess is that the secret service or FBI knew the shooter. What I mean by that is he wasn’t groomed by them but he was on their radar.
Someone who stands to gain from Trump death or attempt at one likely relaxed security for this event and let the shooter take his shot freely.
This is my theory.