There simply hasn’t been any correlation from one period to the next. From Sept. 2 to Sept. 16, for instance, the Republican candidate gained ground in six states, and the Democratic candidate gained ground in six states. But in the next period (from Sept. 16 to Sept. 30), the movement continued in the same direction in only seven of the 12 states.
True, there has been consistent movement in some races. Michigan has very clearly moved away from Republican Terri Lynn Land to Democrat Gary Peters. Arkansas has slowly but surely gone into the red column, and Republican Tom Cotton is a clear favorite there now. But those are the only two races though where one candidate has gained in every two-week period.
Other races have seen reversals of “momentum.” Republican Cory Gardner saw a number of unfavorable polls from Sept. 2 to Sept. 16. His chance of winning dropped from 53 percent to just 31 percent. Since that point, he’s gone on a tear and gained in each of the last three two-week periods. He’s now a 77 percent favorite.