It's a challenge that has befallen previous federal prosecutors and law enforcement officials, to the point that they now typically refrain from taking prominent actions that could have political ramifications within 60 days of a major election.

But there are no clear-cut rules, and both action and inaction can be viewed in hindsight as being politically motivated, something former FBI Director James Comey knows all too well.

"It's a double-edged sword," said Stephen Vladeck, a University of Texas law professor. "There comes a point where you are damned if you do, damned if you don't." 

There is broad speculation that President Trump or someone in his inner circle — including his eldest son, Donald Trump Jr. — could be implicated in Mueller's probe.