A model-based evaluation of the contested Massachusetts State Senate races
Just over half of the Massachusetts State Senate seats—22 out of 40—have a contested general election race, with a likely safe GOP hold in the First Plymouth and Norfolk District represented by Senator Patrick O’Connor (R-Weymouth), and Senator Jamie Eldridge (D-Acton) in a redistricted D+20 Middlesex and Worcester District that is unlikely to end with a Republican pickup.
We will use a model-based approach to understand the competitive landscape of all of the contested State Senate races with respect to those extremes using a simple regression model trained with historical data to provide an estimated range of likely outcomes.
The Bayesian regression model used for this analysis is described in detail in the MA 2022 State Representative Ratings. Briefly, the model has three variables: a measure of partisanship (the Partisan Voter Index), an incumbency factor (Democratic incumbent, no incumbent, or GOP incumbent), and an adjustment for presidential election years. The model is trained with 424 state legislative elections from 2008 through 2021 and explains a little over 70 percent of variance in the Democratic percentage of the vote. The other 30 percent of the variance is due to important factors like candidate and campaign quality, which will determine the results in competitive races. The general, fundamentals-based nature of the model is also why these ratings should only be used to understand the competitive landscape rather than as a prediction of race results.
As an example, let’s take the Hampden, Hampshire and Worcester race between sitting State Representative Jake Oliveira (D-Ludlow) and business owner William Johnson (R-Granby). The Partisan Voter Index (PVI) of the redistricted Hampden, Hampshire and Worcester district is D+4 (3.50 with more precision) meaning that over the last two presidential elections the district is 3.5 points more Democratic than the country as a whole. The incumbency variable is set to no incumbent as State Senator Eric Lesser (D-Longmeadow) ran for Lt. Governor, rather than for re-election. The presidential election variable is false, as it is for all the 2022, off-year races.
Using the trained model to probabilistically simulate the election several thousand times (sampling from the posterior distribution in technical parlance) we see the Oliveira winning 59% of the time, and Johnson winning 41% of the time. If we look at the simulated Democratic percent of the vote, rather than the win/loss percentage, we see an average Democratic percent of 52% of the vote—making this the most competitive State Senate seat with respect to this model. We can also generate a 90% confidence interval for the margin with the Democratic percentage varying from 41% to 63%—a wide enough interval to allow for a comfortable GOP win, or a large Democratic victory. The most likely outcome will be a reasonably narrow victory for Oliveira or Johnson.
For each race we show the candidates, district Partisvan Voter Index (PVI), and the output of the model: a Democratic win probability, and a projected Democratic one-way margin (points above 50/50 tie) with a 90% confidence interval. It is important to remember that this fundamentals-based model does not take into account particulars of each race including important factors like candidate quality and campaign efforts and outreach, which will actually determine the results in competitive races.
In races with a Republican, a third-party, unenrolled, or independent candidate, and no Democratic candidate, the Democratic win percentage and margin should be attributed to the non-Republican candidate.
The highlighted races in the table have both candidates with at least a 10% chance of winning and non-trivial margins on both sides of the break-even margin line.
| Contested State Senate race ratings | |||||||
| district | PVI | Incumbent | D | R | other | dem_win_prob | margin |
|---|---|---|---|---|---|---|---|
| First Plymouth and Norfolk | D+9 | GOP | Stephens (D-Hanson) | O'Connor (R-Weymouth) | 8% | ||
| First Essex and Middlesex | D+10 | GOP | Tarr (R-Gloucester) | Cudney (I-Gloucester) | 10% | ||
| Hampden, Hampshire and Worcester | D+4 | Oliveira (D-Ludlow) | Johnson (R-Granby) | 59% | |||
| Third Bristol and Plymouth | R+1 | Dem | Pacheco (D-Taunton) | Collins (R-Taunton) | 80% | ||
| Worcester and Hampshire | EVEN | Dem | Gobi (D-Spencer) | Amorello (R-Holden) | 82% | ||
| First Bristol and Plymouth | D+1 | Dem | Rodrigues (D-Westport) | Protentis (R-Lakeville) | 85% | ||
| Plymouth and Barnstable | D+4 | Dem | Moran (D-Falmouth) | Macrae (R-Bourne) | 91% | ||
| Hampden and Hampshire | D+6 | Dem | Velis (D-Westfield) | Calabrese (R-Agawam) | 93% | ||
| Second Essex and Middlesex | D+6 | Dem | Finegold (D-Andover) | Defranco (R-Haverhill) | 94% | ||
| Worcester and Middlesex | D+8 | Dem | Cronin (D-Lunenburg) | Hoyt (R-Westford) | 95% | ||
| Bristol and Norfolk | D+8 | Dem | Feeney (D-Foxborough) | Chaisson (R-Foxborough) | Saylor (W-Mansfield) | 96% | |
| First Worcester | D+17 | Kennedy (D-Worcester) | Mair (U-Berlin) | 95% | |||
| Norfolk and Plymouth | D+9 | Dem | Keenan (D-Quincy) | Innes (R-Hanover) | 96% | ||
| Berkshire, Hampden, Franklin and Hampshire | D+18 | Mark (D-Becket) | Phair (U-Pittsfield) | 95% | |||
| Norfolk, Plymouth and Bristol | D+11 | Dem | Timilty (D-Milton) | Muello (R-Braintree) | 97% | ||
| Norfolk, Worcester and Middlesex | D+12 | Dem | Rausch (D-Needham) | Dooley (R-Wrentham) | 98% | ||
| Second Essex | D+12 | Dem | Lovely (D-Salem) | Anketell (R-Peabody) | 98% | ||
| Cape and Islands | D+12 | Dem | Cyr (D-Truro) | Lauzon (R-Barnstable) | 98% | ||
| Third Essex | D+13 | Dem | Crighton (D-Lynn) | Sulustri (I-Swampscott) | 98% | ||
| Second Plymouth and Norfolk | D+14 | Dem | Brady (D-Brockton) | Gordon (R-Hanson) | 98% | ||
| Fifth Middlesex | D+16 | Dem | Lewis (D-Winchester) | Dombroski (R-Wakefield) | 99% | ||
| Middlesex and Worcester | D+20 | Dem | Eldridge (D-Acton) | Christakis (R-Wayland) | 99% | ||