How is the Mueller Investigation Partisan?

I've been seeing an increasing number of Republicans, and even a legal scholar claim the investigation is a partisan attack.

The investigator, Robert Mueller, is a lifelong Republican and war hero with several medals and an impeccable record. Not to mention he's literally only two levels away from Trump in the Chain of Command.

So how is this partisan? Is there any logical basis for this? Or is this Trump's "I am not a crook" moment?