My analysis last Sunday showed control of the Senate to be a toss up at 48% probability for Democrats and 52% for Republicans. The expected outcome was a 49 to 51 split in favor of the Republicans.
Since then, I’ve added something exceeding 50 polls to the mix. As we saw in the Presidential race, the polls since last Sunday have tended to be more favoriable to Republicans.
Today, after 100,000 simulated elections, Democrats have a Senate majority 1,762 times, and I’ll assume they control the Senate with the 10,293 ties. The Republicans control the Senate 87,945 times. This suggests Democrats have a 12.1% probability of controlling the Senate and Republicans have a 87.9% probability of controlling the Senate. So, indeed, the Democrat’s chances have fallen.
The Republican candidate has primarily gained in five states.
In Indiana, Democrat Evan Bayh’s once solid lead has vanished. Last Sunday, he had an 88.5% probability of defeating Todd Young (R). Today he would win with a 24.1% probability. The polling picture pretty much tells the story.
Sen. Roy Blunt (R) has strengthened his lead over Jason Kander (D) in Missouri. Last analysis Blunt was at a 77.8% probability of winning, but five new polls, including two ties and three with Blunt up, puts the probability at 92.8% today. On the other hand, two months ago, there was little evidence that Blunt would lose.
In Nevada, five polls aged out, and these mostly favored Catherine Cortez Mastro (D) over Joe Heck (R). Heck leads in four of the five current polls. Consequently, the Republican has gone from a 54% to a 93% probability of winning today.
The story is somewhat similar in New Hampshire. Four polls aged out, including one that had Maggie Hassan (D) up by +9 over Sen. Kelly Ayotte (R). The new polling has mostly been more favorable to Ayotte. We now have ten current polls, six that favor Ayotte, two that favor Hassan, and two ties. The net result is that Ayotte has gone from a 67% chance to an 86% chance of winning an election today.
In Wisconsin, four polls aged out, including one with Russ Feingold leading Sen. Ron Johnson by +12%. The four current polls have Feingold up by +8%, +5%, +1%, and +2%. As a result, Feingold’s chances have dropped from 99.8% to 85.9%.
Democrats have made a couple of notable gains.
The North Carolina senate race largely favors Sen. Richard Burr (R) over challenger Deborah Ross (D). But the last couple of days have seen one tie and two polls that favor the Democrat. The result is that Burr has fallen from a 90% probability of winning last Sunday to an 86% chance today.
The biggest news for Democrats happens in Pennsylvania, where Sen. Pat Toomey (R) is being challenged by Democrat Katie McGinty. Seven polls aged-out, and these polls were pretty much a toss-up between the two candidates. Eight new polls have been added that largely favor McGinty. With the four carry-over polls, we now have two current polls that favor Toomey, eight that favor McGinty, and two ties. As a result, Pennsylvania has flipped from red to blue. McGinty was at a 45.8% probability of winning last Sunday, and she would win a race today with a 99.7% probability.
Here is the distribution of Senate seats from the simulations:*
This graphs shows the probability of at least each number of seats controlled by the Democrats:*
- 100000 simulations: Democrats control the Senate 12.1%, Republicans control the Senate 87.9%.
- Average ( SE) seats for Democrats: 48.5 ( 0.8)
- Average (SE) seats for Republicans: 51.5 ( 0.8)
- Median (95% CI) seats for Democrats: 48 (47, 50)
- Median (95% CI) seats for Republicans: 52 (50, 53)
Expected outcomes from the simulations:
- Democratic seats w/no election: 35
- Independent seats w/no election: one
- Republican seats w/no election: 30
- Contested Democratic seats likely to remain Democratic: nine
- Contested Republican seats likely to remain Republican: 21
- Contested Democratic seats likely to switch: one
- Contested Republican seats likely to switch: three
This table shows the number of Senate seats controlled for different criteria for the probability of winning a state:* Safe>0.9999, Strong>90%, Leans>60%, Weak>50%
|Threshold||Safe||+ Strong||+ Leans||+ Weak|
This table summarizes the results by state. Click on the poll number to see the individual polls included for a state.
|State||@||polls||size||Democrat||Republican||% wins||% wins|
@ Current party in office
& An older poll was used (i.e. no recent polls exist).
*Analysis assume that the two independent candidates will caucus with the Democrats.
Details of the methods are given in the FAQ.
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