From the heady heights of a 1 in 3 chance of taking the Senate, the odds, as calculated by 538's forecast model, are now down to about 1 in 5 (David Byler's model over at The Weekly Standard has the Democrats' chances at comparatively bullish 1 in 4).
What happened? As cogently summarized by Nate Silver in his recent piece:
"In general, Democrats’ position has gotten worse in red states but has been steady-to-improving in purple and blue states. There are a couple of exceptions to this pattern (Joe Manchin has continued to poll well in West Virginia) but not many.
On top of that, Democrats have had particular problems in North Dakota, where Heitkamp has seen her numbers go especially south."
The Heitkamp collapse is vividly illustrated by the graphic below from Silver's article.
Meanwhile, in House-land things continue to look good. Nathaniel Rakich notes, also on 538:
"The situation is...dire for the GOP in the House, where Democrats have a 7 in 9 chance (78 percent) of taking control. They are so dire in some GOP-held districts, in fact, that national Republicans have begun pulling their resources or never invested them in the first place — effectively ceding those seats to Democrats, presumably so that the GOP can bolster more winnable districts."
The probability of a split outcome between House and Senate appears to be getting higher and higher.