The problem is that criminal laws can't be applied,
even in principle, without the exercise of prosecutorial discretion. Criminal laws are written broadly and legislatures trust prosecutors to apply the laws to the subset of conduct that falls within the arguable letter of the law. That makes prosecution of political figures incredibly dangerous, because prosecution is
inherently a discretionary act.
Consider 18 USC 1343:
> Whoever, having devised or intending to devise any scheme or artifice to defraud, or for obtaining money or property by means of false or fraudulent pretenses, representations, or promises, transmits or causes to be transmitted by means of wire, radio, or television communication in interstate or foreign commerce, any writings, signs, signals, pictures, or sounds for the purpose of executing such scheme or artifice, shall be fined under this title or imprisoned not more than 20 years, or both.
A politically motivated prosecutor willing to stretch phrases like "property" and "false ... representations" could use 18 USC 1343 to turn minor infractions into federal felonies with the prospect of 20 years in prison. It's entirely up to the prosecutor to ensure that the atomic bomb of 18 USC 1343 is applied only to conduct that actually warrants such extreme charges.
I'd note that, had Jim Comey been willing to push the law as far as the words would allow, he would've had a legal basis to prosecute Hillary Clinton for felonies relating to handling of classified information: https://www.fbi.gov/news/press-releases/statement-by-fbi-dir.... Comey's statement in connection with the decision to recommend no charges is instructive:
> Although there is evidence of potential violations of the statutes regarding the handling of classified information, our judgment is that no reasonable prosecutor would bring such a case. Prosecutors necessarily weigh a number of factors before bringing charges. There are obvious considerations, like the strength of the evidence, especially regarding intent. Responsible decisions also consider the context of a person’s actions, and how similar situations have been handled in the past.