The fate of the most consequential cryptocurrency legislation in a generation now rests on a handful of undecided senators, as the digital-asset industry and its banking-sector opponents wage an unprecedented grassroots campaign across the home states of lawmakers during the August congressional recess. With a procedural vote scheduled for September 15, both sides are racing to lock down the 60 votes needed to advance the CLARITY Act, a market-structure bill that would define how digital tokens are regulated in the United States.

Coinbase policy chief Faryar Shirzad told industry advocate Scott Melker in a September 6 conversation that the bill faces two plausible outcomes: it could fall just short of the cloture threshold, or a late…