The process is scientific, if applied correctly. It's basic statistics. And the scientific processes apply whether one is speaking of political races, consumer surveys or favorite football teams. If they were not reliable, you would not see so many polls coming up with similar ranges. You'd have one poll showing Harris leading 63-34, another with Nelson leading 58-29, etc.
And even if done perfectly, a poll is considered only 95% accurate (again based on statistics).