Nobody is saying that running for president magically makes Joe Biden immune from investigation or prosecution. if reasonable suspicion or probable cause existed that Joe Biden had committed a crime, the matter would be discussed discretely between investigators and prosecutors. if the president had even an ounce of discretion or scruples, as each of the previous 44 presidents seems to have had, the president would avoid attending any meetings about the matter. He would not want to be accused of starting the investigation merely to spy on the opponent's strategy. Remember that? Trump still says that Obama wanted to spy on him in 2016, but no one has shown any evidence that anybody in the FBI leaked anything about Trump's political traategy back to Obama or back to Clinton.
In any case, Trump wants to have it both ways - he wants to insist that either Obama or Clinton used an FBI investigation to spy on his campaign, and that this was wrong, and he also wants to insist that he has every right to do the same thing to Joe Biden, personally involving himself in the investigation

---