Technically, no. This is the reason that the Colonel-Commissar rank - which does exist - is very, very rare, and held usually in only extraordinary cases, and apparently referring to a Commissar who is awarded regimental command rank in the Imperial Guard itself, rather than the Departmento Munitorium. Though the records show the rank existing relatively recently (Sabbat Worlds Crusade, in the form of Ibrahm Gaunt) the inherent clash between the two roles is normally seen as a threat to a regiment's discipline - Commissars maintain discipline and watch commanders, the commander leads the regiment in combat - the two roles are technically exclusive.



Granted, he could be acting as commanding officer if he's had reason to execute the commanding officer for cowardice or incompetence. But that role basically exists only until a suitable replacement can be found.