Just like the rest of us, special prosecutor Robert Mueller is apparently concerned that Trump may pardon his closest associates or even himself and, to that end, Mueller has hired one of the most qualified people in the country to pardon-proof his investigation.
He’s Michael Dreeben, a bookish career government lawyer with more than 100 Supreme Court appearances under his belt.
Acting as Mueller’s top legal counsel, Dreeben has been researching past pardons and determining what, if any, limits exist, according to a person familiar with the matter. Dreeben’s broader brief is to make sure the special counsel’s prosecutorial moves are legally airtight. That could include anything from strategizing on novel interpretations of criminal law to making sure the recent search warrant on ex-campaign adviser Paul Manafort’s home would stand up to an appeal.
It seems reasonable to infer that Mueller would not be this concerned about Trump's pardon power unless he expects to issue indictments. It appears to be a matter of when, not if.
It's been reported in the past that Mueller is working closely with state attorneys general, including New York AG Eric Schneiderman, to determine if any state laws have been broken. Trump can't pardon his friends for breaking state laws, only federal laws.