down-to-the-wire

The phrase 'down-to-the-wire' is a distinctly American idiom originating from horse racing in the late 19th century. It refers to a race so close that the outcome is not determined until the horses reach the finish line, which was traditionally marked by a stretched wire. The literal image is of a contestant racing 'down' the track all the way 'to the wire.' This vivid, concrete metaphor from the world of sport was then abstracted and applied to any intensely competitive or suspenseful situation that remains unresolved until the final, critical moment, capturing the tension of a photo finish.

This is a hyphenated compound idiom. It is not productively formed from separable morphemes in the modern sense but is a fixed phrase derived from a specific metaphorical context.