Campaigns over the past month have been trending toward the Republicans in Arkansas, Alaska and Louisiana, and the odds of a Republican upset, though still fairly long, have also risen in Michigan and New Hampshire. To some extent, Mr. Orman’s earlier gains obscured these trends. Now that these gains may be slowing or reversing, the full effect of the erosion in the Democratic position is starting to be felt. Thus in many ways, the latest increase in the G.O.P.'s overall chances is less of a surge and more of a return to normal.

Still, the race has three weeks remaining, and perhaps the most notable aspect of this election cycle (from a forecasting perspective at least) is the uncertainty. The most competitive races are also those surrounded by the biggest question marks. Forecasting models are designed to predict the future by analyzing past data. As a general rule, the less relevant that the past data is, the less certain you can be that your model still makes sense, and a race like Kansas’ falls well outside the scope of past data.

Races in Alaska and Louisiana bring their own challenges. Alaska has a history of polling difficulties, and there has been but one nonpartisan, live-interview poll of the state this cycle. Recent polling has shown the Republican, Dan Sullivan, with a consistent four-to-six-point lead, but questions remain about how much stock to put in those numbers.