Bellwether Wikipedia
OED
4.b.1876 Something that establishes or signals a widening trend; an indicator or predictor of the way in which something will develop or change. Frequently as a modifier, as in bellwether county, bellwether state, etc. Originally and frequently in political contexts.1876 Ohio, the bell-wether State of the Union, is a ‘close’ state... The Ohio election takes place three weeks in advance of the National election. Cincinnati Enquirer 15 January 4/2
1908 If the dispensary is not a success here it will not be a success anywhere. Craig will be a fine bell-wether county. Vinita (Oklahoma) Leader 9 April
1968 The number of robberies—the offense that crime experts consider the bellwether of a nation's tendency toward criminal violence. International Herald Tribune 3 September 7/3
1993 Steinem, always a bellwether of the going trend, aligns the passion for witchcraft with feminist political doctrine. Harper's Magazine July 7/2
2024 Nuneaton is traditionally a bellwether constituency in general elections—when the Tories held it in 2015 it was an early sign that they had defied expectations to win a majority. Guardian (Nexis) 5 April (Politics section) 5