Election Eve 2020: The Senate Shifts Slightly to the Right

Senate Democrats*	Senate Republicans
greater than 99.9% probability	less than 0.1% probability
Mean of 53 seats	Mean of 47 seats

The previous analysis of the Senate races four days ago showed control of the Senate likely to go to the Democrats, with an expected 54 seats. Since then there have been many new polls released. As a result, the race has tightened slightly.

Now, the Monte Carlo analysis using 100,000 simulated elections gives Democrats have a Senate majority 99,773 times and produces 227 ties. Republicans never control the Senate. That is an almost imperceptible “slide” for the Democrats from the previous analysis. But the mean number of seats has dropped by one from 54.2 to 53.2. Part of the reason is that I’ve tightened the “current poll” window from 1 month to 3 weeks, as there is enough polling to justify the narrower window. And doing so increases the chances of catching late trends in the race.

There were a few big changes. In Alaska, where Democrat Al Gross is attempting to unseat Republican Sen. Dan Sullivan, the newest polls take the race from a tie to Sullivan in the lead with a 78% chance of winning an election today. For Georgia’s seat 1 race where Democratic challenger Jon Ossoff is trying to unseat Republican Sen. David Perdue, the last 6 of 14 polls have put Ossoff in the lead. Perdue had a 65% chance of winning four days ago, but now Ossoff has the edge with an 87% chance of winning. In Iowa, where Democrat Theresa Greenfield is challenging Republican Sen. Joni Ernst, Greenfield’s once strong lead has slipped. Four days ago she had a 76 percent probability of winning. Now she has a 61% chance. The open seat in Kansas has Democrat Barbara Bollier head-to-head with Republican Roger Marshall. Boiller had a slight lead that translates to a 63% probability of winning. But the new polling has Marshell ahead with a 91% chance of winning.

The Race in Maine has tightened as well. Democrat Sara Gideon has been leading Republican Sen. Susan Collins. In fact, Gideon still leads in all three current polls, but one poll is nearly a tie. Gideon’s lead has slipped from 97% a few days ago to 87% probability today. In another challenge to an incumbent, Democrat Mike Espy is challenging Republican Sen. Cindy Hyde-Smith for her Mississippi seat. The polling tightened up recently, and Hyde-Smith had a 56% chance of winning a few days ago. But a new Civiqs poll has Hyde-Smith up by +8%, boosting the Senator’s chances to a 90% probability of winning. Finally, in the South Carolina race, Democrat Jamie Harrison’s challenge of Republican Sen. Lindsey Graham has seen a shift in Graham’s chances of prevailing from 56% to an 88% chance.

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 100.0%, Republicans control the Senate 0.0%.
• Average ( SE) seats for Democrats: 53.2 ( 1.1)
• Average (SE) seats for Republicans: 46.8 ( 1.1)
• Median (95% CI) seats for Democrats: 53 (51, 55)
• Median (95% CI) seats for Republicans: 47 (45, 49)

Expected outcomes from the simulations:

• Democratic seats w/no election: 34
• Independent seats w/no election: one
• Republican seats w/no election: 30
• Contested Democratic seats likely to remain Democratic: 11
• Contested Republican seats likely to remain Republican: 16
• Contested Democratic seats likely to switch: one
• Contested Republican seats likely to switch: seven

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
Safe Democrat	48
Strong Democrat	2	50
Leans Democrat	3	3	53
Weak Democrat	0	0	0	53
Weak Republican	0	0	0	47
Leans Republican	4	4	47
Strong Republican	3	43
Safe Republican	40

This table summarizes the results by state. Click on the poll number to see the individual polls included for a state.

		#	Sample	Percent	Percent	Democrat	Republican
State	@	polls	size	Democrat	Republican	% wins	% wins
AL		4	3255	43.7	56.3	0.0	100.0
AK		2	1404	48.6	51.4	21.8	78.2
AZ		23	16401	53.0	47.0	100.0	0.0
AR		3	1668	32.3	67.7	0.0	100.0
CO		7	4986	55.1	44.9	100.0	0.0
DE		1&	712	67.8	32.2	100.0	0.0
GA		14	12129	50.7	49.3	86.8	13.2
GA		4	2470	54.9	45.1	100.0	0.0
ID		1&	487	34.1	65.9	0.0	100.0
IL		2	1289	65.6	34.4	100.0	0.0
IA		12	7765	50.2	49.8	61.1	38.9
KS		3	2512	48.0	52.0	9.0	91.0
KY		4	2213	44.3	55.7	0.0	100.0
LA		1&	680	38.8	61.2	0.0	100.0
ME		3	2255	51.6	48.4	86.5	13.5
MA		2	1474	69.2	30.8	100.0	0.0
MI		27	21762	53.5	46.5	100.0	0.0
MN		9	6006	54.5	45.5	100.0	0.0
MS		1	487	45.8	54.2	10.5	89.5
MT		5	3315	49.7	50.3	38.6	61.4
NE		1	395	27.3	72.7	0.0	100.0
NH		4	3090	57.7	42.3	100.0	0.0
NJ		2	1194	65.3	34.7	100.0	0.0
NM		1	1133	54.2	45.8	97.8	2.2
NC		27	22506	51.9	48.1	100.0	0.0
OK		1	5102	39.6	60.4	0.0	100.0
OR		1&	849	61.1	38.9	100.0	0.0
RI		0	0			(100)	(0)
SC		6	5085	48.8	51.2	12.0	88.0
SD		0	0			(0)	(100)
TN		1	551	39.0	61.0	0.0	100.0
TX		17	17674	47.3	52.7	0.0	100.0
VA		6	5022	57.8	42.2	100.0	0.0
WV		1&	386	38.3	61.7	0.0	100.0
WY		0	0			(0)	(100)

^@ 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.

The most recent analysis in this match-up can be found from this page.