Does commander damage carry over between different combat phases in the same turn?

Commander damage is tracked cumulatively over the entire game, not just within a single turn or combat phase. A player loses the game the moment they have been dealt 21 or more combat damage by the same commander over the course of the game (Commander Rule 10, formalized in EDH rules). There is no mechanism that resets or separates this total between combat phases.

This is especially relevant because certain cards and effects can grant a creature additional combat phases in a single turn — for example, Aggravated Assault or Savage Beating. If your commander deals combat damage to the same opponent in multiple combat phases within one turn, all of that damage counts toward the 21-damage threshold. The Comprehensive Rules confirm that state-based actions check this running total continuously (CR 704.5u covers the commander damage loss condition).

It is also important to note that only combat damage counts toward commander damage — damage dealt by a commander through non-combat means (spells, abilities, etc.) does not count (CR 702.124, EDH additional rules). But any combat damage, in any combat phase, on any turn, accumulates.

Example: Your commander (a 7/7) attacks in a first main combat phase and deals 7 commander damage to an opponent. Later that same turn, Aggravated Assault triggers a second combat phase and your commander attacks again, dealing another 7 damage. That opponent now has 14 commander damage on record. If this happened again a subsequent turn for 7 more, they would reach 21 and lose the game.

Bottom line: always track commander damage as a running game-long total. It never resets between phases, turns, or even if the commander changes zones — though damage dealt after a commander returns from the command zone still counts under the same tracker for that commander (EDH rules).