Most bills die. That is the base rate, and any honest passage estimate starts by respecting it: a bill introduced with no cosponsors, no committee action, and no companion in the other chamber is overwhelmingly likely to go nowhere, regardless of how consequential it would be.
What moves the number
- Committee action — a markup is worth more than a hundred cosponsors.
- Cosponsor breadth across parties, not raw count.
- A companion bill moving in the other chamber.
- Whether the text has been folded into a larger vehicle.
- Prediction-market pricing, where a liquid market exists.
Where it should not be trusted: anything attached to an appropriations deadline, anything in the last six weeks of a session, and anything where the substance has quietly moved into a different bill number. In those cases the estimate is measuring the wrong object, and we say so on the bill page rather than letting the number stand alone.
A forecast that never says "I do not know" is not a forecast.