Bellwether state:

A state that historically tends to vote for the winning candidate, perhaps because it is, demographically, a microcosm of the country as a whole. A good example is Ohio, which has not backed a losing presidential candidate since 1960.

The term derives from the name for a sheep which shepherds would fit with a bell. By listening out for this sheep, the bellwether, shepherds were able to locate the position of the entire flock.