当政

Formal/written. News and history texts use 当政 for heads of state, party leaders, and dynastic emperors - 'someone is currently in office.' Neutral in tone (neither glorifying nor critical). For everyday 'in charge,' use 掌权 or 执政; 执政 specifically applies to ruling parties.

grammar

Functions as both a verb ('to govern') and as a temporal frame: X 当政期间 / X 当政时 = 'during X's tenure in power.' Common in academic writing about specific eras.