(Updated to reflect the effect of Senator Stevens’s conviction today. A Begich win over Stevens would make a 59-41 split even more likely. -Sam)

The voting makeup of next year’s Congress is a simple sum of votes, and therefore poses an easier estimation problem than the Presidential race. Many other sites give just the average estimate. Here I will give a probabilistically-based analysis that (a) estimates the likelihood of Democrats reaching a supermajority in the Senate, and (b) puts confidence intervals on the estimates. This projection will be updated as the election nears.

The Senate. Based on Pollster.com data, Democrats are headed for new wins or continued control in 57 seats (counting Sanders and Lieberman); Republicans, 38 seats. The remaining five races are uncertain: Alaska, Georgia, Kentucky, Minnesota, and Mississippi (B). Two states are polling-poor: In Alaska, the third-oldest poll is from 10/6, a long time in the corruption trial of Senator Ted Stevens (R). In Mississippi, the third-oldest poll is from 9/24. So the main weakness in Senate meta-analysis is that polls lag behind current conditions.

The rule used here is the same as the Presidential meta-analysis: start with the last 3 polls and last week of polling, then calculate a z-score, which is converted to a probability. I use the t-distribution rather than a normal distribution, which statistics aficionados may appreciate. To allow for added uncertainty, I assume that between the last available polls and Election Day, the margin may change by an unknown amount S, with an S of up to +/-2%.

Under these assumptions, the win probabilities are

State	Dem	GOP	Median margin	Dem win probability
Minnesota	Franken	Coleman	D +3%	91%
Alaska	Begich	Stevens	D +1%	71%
Mississippi	Musgrove	Wicker	R +2%	17%
Georgia	Martin	Chambliss	R +2%	8%
Kentucky	Lunsford	McConnell	R +4%	3%

The resulting probability distribution of Senate outcomes is:

Current Senate predictions: 58-60 Dem/Ind, 40-42 GOP. The probability of falling within this range is 92% (96% w/Begich). The most likely outcome is 59-41, a pickup of 8 seats, with a probability of 49% (69%), approximately even odds. The probability of reaching 60 or more seats is 21% (23%), or 4-1 against.

Note: After I wrote this, Senator Stevens’s conviction on ethics charges was announced. However, the above conclusions are basically unaffected. The figures in (italics) above indicate the case in which Begich wins.

A moderate swing could alter these odds. It would take a swing of about 2 percentage points for the Democrats to reach 60 seats. Such a swing could come from 2% of new commitments to the Democratic candidate from among undecideds, or from 1% of voters switching sides. I will update this calculation when new polls come in.

Previously I recommended particular races as being on a knife edge, but conditions have now shifted. In terms of resource allocation, current conditions indicate the following categories:

Lean Republican. These races are aggressive investment opportunities for Democrats, conservative for Republicans: Georgia (Martin v. Chambliss) and Kentucky (Lunsford v. McConnell).

Knife edge. Giving to this race provides the maximum leverage for both sides: Mississippi-B (Musgrove v. Wicker) and Alaska (Begich v. Stevens). They are listed on the ActBlue site and at the NRSC.

Lean Democratic. This races is a conservative investment opportunity for Democrats, aggressive for Republicans: Minnesota (Franken v. Coleman). This race is no longer on a knife edge.

The House. Here polls are sparse, as few as one per competitive district. However, even if we cannot predict individual races, we can pool all districts to estimate the overall outcome and its uncertainty. This approach worked quite well in 2006.

Today’s data at Pollster.com give 158 strong R, 8 lean R, 24 toss-up, 10 lean D, and 235 strong D. Democrats lead in 13 out of 24 toss-up races. Therefore the median expectation is 158+8+11= 177 Republican seats, 13+10+235=258 Democratic seats. If we assume that the 18 “leaner” seats have win probabilities of 0.8-0.9, and toss-ups have win probabilities of 13/24 or 11/24, binomial math gives a snapshot with confidence intervals (CIs):

Democrats 258 seats (68% CI 255-261, 95% CI 252-264),

Republicans 177 seats (68% CI 174-180, 95% CI 171-183).

The 2006 Congress came in at 233-202 and is now 235-199 (1 vacancy). The pickup for Democrats is therefore 23 seats (68% CI 20-26 seats, 95% CI 17-29 seats).

One benefit of calculating confidence intervals is that it provides a means of estimating the probability of particular events such as whether Democrats will get 271 or more seats in the House. Such events are traded on InTrade and other electronic markets. However, these sites only approximate the true odds. There are discrepancies, which I will write about later.