Congress has convened 10 times since 2000. Over this period Republicans have held the majority in the Senate six times, while Democrats have been in the majority four times. All four periods of Democratic control came between 2007 and 2015, with Democrats controlling as many as 57 seats between 2009 and 2011. However, Republicans were able to regain control of the Senate in 2015 and have held it ever since.
As of October 30 the model is predicting that the most likely outcome in the Senate is maintenance of the status quo — Republicans holding 51 seats and Democrats occupying 49. However, due to the wide range of possibilities, this outcome still has relatively low probability with a 17.4% chance. Interestingly, the model predicts that any outcome that falls between Democrats gaining two seats to Democrats losing three seats has a greater than 9% chance of happening.
Cook uses four different qualifiers to rate races: solid, likely, lean, or toss-up. If a seat is solid Republican or Democrat, Cook judges that it is highly likely that it will be won by that party. Conversely, if a seat is a toss-up, either party has a good chance of winning. Currently, Republicans have a slight edge as five of the nine toss-ups are for seats currently held by Democrats. Furthermore, Cook rates the North Dakota Senate Race, a seat currently held by Democrat Heidi Heitkamp, as lean Republican. In recent weeks Cook made headlines when it moved New Jersey Democratic incumbent Robert Menendez’s seat into toss-up. This comes despite strong polling for Menendez, who currently has a Real Clear Politics average lead of 6.5% in the polls and has a 90% chance of winning according to FiveThirtyEight.
As of October 30 Heitkamp, was down an average of 14% to her Republican challenger Kevin Cramer. The last poll to give Heitkamp an advantage came in February. On the other side of the aisle, while Dean Heller currently has an average lead of 1.7%, the race is effectively a dead heat, as his lead over Democratic challenger Jacky Rosen falling well within the margin of error.