Can you use the commander damage rule to win by dealing 21 damage across multiple different combat steps?

In the Commander format, a player loses the game if they have been dealt 21 or more combat damage by a single commander over the course of the entire game. This is tracked cumulatively — it does not reset between turns or combat phases. The relevant rule is Commander rule 903.10a.

The damage does not have to come from a single attack or even a single turn. As long as it is combat damage dealt by the same commander to the same player, every point counts toward that player's running total for that commander. So if your commander deals 7 damage in turn 5, 7 more in turn 8, and 7 more in turn 12, that opponent has taken 21 commander damage and loses immediately when the last 7 is dealt.

It is important to note that only combat damage counts toward the 21-damage threshold. Damage dealt by a commander through non-combat means — such as a Lightning Bolt copied by a commander, or commander damage redirected via abilities — does not count. Only damage assigned during the combat damage step (CR 510) qualifies.

Each commander is tracked independently, and each opponent tracks their own total against each of your commanders. In a multiplayer game, you could theoretically defeat multiple opponents with commander damage from the same commander, but each opponent's tally is separate.

Example: You attack with your 7/7 commander on turns 3, 5, and 7, and your opponent blocks ineffectively each time, taking 7 combat damage each turn. After the combat damage step on turn 7, that opponent has accumulated 21 commander damage from your commander and immediately loses the game per rule 903.10a.