Oracle Text

Counter target spell that targets a creature.

Card Rulings

10/4/2004 If a spell is modal, use the target(s) of the current mode only for this check. Targets for the other modes are never selected.
10/4/2004 Intervene can only target a spell that is currently targeting a creature.
10/4/2004 It means “targeting at least one creature” not “exactly one creature”.
10/4/2004 If the spell’s target leaves the battlefield before this spell resolves, Intervene won’t resolve since it no longer targets a spell that targets a creature.