How big is the boost for the Conservatives from the new Parliamentary boundaries?
Colin Rallings and Michael Thrasher have their authoritative analysis of the new Parliamentary constituency boundaries out, and the impact of the changes on parties.
The key thing to remember about their analysis is that it doesn’t, indeed can’t, take into account how parties would have campaigned differently if the new boundaries had been in place at the last general election. This particularly matters for parties like the Liberal Democrats where the level of campaign activity varies massively from seat to seat, and so an area that got no campaigning last time might have got a by-election style level of campaigning had the boundaries been different.
Even so, the Thrasher and Rallings figures provide a handy basic benchmark, particularly for the larger parties. It’s notable how limited the boost to the Conservatives is from the new boundaries, especially given that they need all they help they can get at the moment.
See also this analysis from John Curtice.
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I get the impression that R&T’s ward-level local election data that they use to calculate the effects of boundary changes assumes those electors will vote the same way in a GE that they do in local elections (they do include a caveat). I suspect that this is probably the largest systematic error in their model. For example, the Liberal Democrats have controlled Watford Council for over 20 years (and currently have over 75% of the councillors as well as the elected Mayor) but have only once come close to winning the Watford Parliamentary seat.
I’d be interested to see a backward look on boundary changes – i.e. an analysis next year of how the 2024 election would have panned out on 2019 boundaries.
Understandably, it’s always done the other way around. But the backward look would check if the projections are even worth doing.
I recall before 1997 that Conservative boffins were widely deemed to have played a blinder in that boundary review, creating a bunch of new notionally Tory marginals by carving votes out of safe Tory neighbours. But, as it transpired, the Conservatives won not a single one of the notional marginals they had lovingly created, while some of the supposedly safe neighbours bitterly regretted giving up reliable voters.