Use the word bellwether to indicate someone or something that takes the lead in a group or movement.
Literally, a bellwether is a sheep (often wearing a bell) that leads a herd. Unless you herd sheep, you probably don't see many bellwethers that say "Baaa." That's the origin of this word, but there are many other types of bellwethers who lead the way in different areas. Apple was a bellwether in technology. Rosa Parks was a bellwether in civil rights. And, during election season, those "bellwether states" are the ones where the primaries seem to predict the outcome of a party's nomination.