Up to the very end; up to the finish line

Trust me! Do whatever is right and I'll stand up with you down to the wire

Fidati di me! Fai le cose giuste e io starò al tuo fianco fino in fondo

It's been a very exciting race: they went head-to-head down to the wire

È stata una gara entusiasmante: un testa a testa fino all'ultimo

This phrase refers to races where the winner is determined by whoever crosses the finish line first. In horse races a real wire is stretched across the finish line, while in human races the so called "wire" is a ribbon, a string or a tape. It's meant to help the judges see clearly who crosses first in a close race