As votes are tabulated in the races that will determine control of the Senate, the speed at which votes are counted can vary widely within each state. That can lead to changes in who is leading the race at a particular point in time.
The Associated Press will declare a winner in a race when it is determined there is no scenario that would allow the trailing candidate to close the gap. That decision is informed by votes that have already been tallied, results from the AP VoteCast survey of why voters voted how they did, and an estimate of the total votes expected to be cast. The expected vote total is determined by AP’s proprietary research, knowledge and expertise; it will vary over the course of the election tabulation, based on the incoming total votes at any given time for the race.