You’ve got a problem there: The DNC and the RNC (and any other political party) are pretty much private actors who are in no way affiliated with a governmental (US, local, state) entity. Each gets to decide who it wants to represent it. Favoring one candidate over another isn’t really an issue. Heck, they used to meet in closed-door sessions and just nominate their chosen guy to run for President. Having Congress investigate either of those entities for alleged malfeasance during their own party primary would be akin to investigating a local Elks Club for favoring Elk #1 over Elk#2 in a running to see which Elk gets to be lodge leader (or whatever they call it) of said Elks Club.
Malfeasance in the general election is a different matter, of course.