All races have tightened up. This is because they poll democrats more heavily closer to election time. It makes it appear that the race is close, encourages the democrats, and demoralizes the republicans. The race then gets to within a close enough margin that cheating can swing the rest of the votes to the desired candidate. It may not be discussed openly, but it is “understood” by the democrat leaders and their friends in the media, who are responsible for reporting on the polls and not reporting on the fraud.
Another reason for polls that cheat to Dem side is vote fraud. If the polls were to favor the GOP by a fairly decent margin and the Dems wind up winning, questions may arise. If the results mirror the polls, you can claim the FOP voters did just not want to show up.