Mueller is investigating coordination between the Trump campaign and the Russian government, the acceptance of information of value from the Russian government, and the acceptance of money from the Russian government.

This is in additition to decades of Trump crime family NY and NJ crimes, their mafia casinos used by Russians et al for massive money laundering, and their more recent money laundering international escapades like Trump Toronto or Azerbaijan (where the Trump org worked directly with the Iran Rev National Guard -- denoted a terrorist organization today!) 

Neither Trump nor Clinton committed any crime engaging in oppo research. 

The crime was a campaign accepting things of value from a foreign government to alter the outcome of the election.

As the Russian spy told Don Jr before the treason meeting: "This is obviously very high level and sensitive information but is part of Russia and its government’s support for Mr. Trump"

Government support = Election crime.