What would be the electoral arithmetic for pro‑independence parties if the SNP withdrew from regional lists in a specific Scottish region?
Executive summary
Withdrawing the SNP from regional (list) ballots would change the mechanics of Additional Member System seat allocation but does not automatically guarantee pro‑independence gains or losses: the result depends on how SNP list votes redistribute between smaller pro‑independence parties (Greens, ISP, Alba) and how many constituency seats the SNP still wins in that region, because the AMS uses constituency wins to reduce a party’s entitlement to list seats [1] [2]. Campaign analysis from the Independence for Scotland Party argues that a rival pro‑independence list party would need to reach “critical mass” (roughly 15% of SNP list votes, about 7% of the total) to materially change outcomes, and warns that under some scenarios a fall in SNP constituency success combined with a regional vote drop could cost a pro‑independence majority [3].
1. How the maths works: AMS and the role of constituency wins
The Scottish Parliament uses the Additional Member System: voters cast a constituency (FPTP) vote and a regional list vote, and the allocation of the regional seven seats compensates for constituency results so overall representation is roughly proportional [1] [2]. That means if the SNP did not stand on the regional list, they would still win or lose constituency seats as before, but their absence on the list would lower the number of votes and therefore divisors applied to pro‑independence totals in the region — potentially freeing list seats for other parties [1] [2].
2. Where redistribution helps pro‑independence parties — and where it doesn’t
If SNP list votes transfer mainly to other pro‑independence list parties — for example the Greens, ISP or Alba — those parties could pick up the seats the SNP would otherwise have won on the list; ISP’s own analysis suggests that replacing SNP list MSPs with an ISP MSP could preserve a pro‑independence majority if the party reaches sufficient share [3]. However, the multiplicity of Unionist parties (Labour, Conservatives, Lib Dems) gives them an advantage under list divisors because their votes aren’t concentrated in one party, which can result in Unionist parties picking up more compensatory seats unless a second strong pro‑independence list party attains critical mass [3].
3. Historical patterns that illuminate risks and opportunities
Past regional outcomes show how extremes in constituency dominance can wipe out list representation: North East Scotland in 2016 gave SNP nine of ten constituencies and no list seats for the party in that region — an illustration of how constituency success reduces list returns [3]. By contrast, in 2021 pro‑independence representation depended on both SNP constituency performance and Green list gains, with the combined pro‑independence grouping winning 72 of 129 MSPs overall [4] [5]. Those precedents show withdrawing the SNP from lists can either shift comparable seats to allies or leave gaps if transferred votes are too fragmented.
4. Key quantitative thresholds and a warning scenario
ISP’s public modelling sets a benchmark: an effective secondary pro‑independence list needs around 15% of what SNP list voters currently provide (roughly 7% of total regional vote) to materially compensate for an SNP list absence [3]. Their scenario also flags a danger: if the SNP simultaneously lost about ten constituency seats and saw a 5% regional vote fall, the compensatory list calculus could eliminate a pro‑independence majority — because the SNP would not pick up enough list seats to offset constituency losses [3]. This illustrates that withdrawing from lists is not a simple zero‑sum transfer of seats.
5. What cannot be concluded without regional vote data
Any precise arithmetic for a particular region requires the actual constituency and regional vote shares and where transfers would flow if the SNP stood down on the list; those numbers are not present in the sourced reporting, so a definitive seat‑by‑seat calculation for a named region cannot be produced here [1] [3]. The balance of risk versus reward turns on voter behaviour (who SNP list voters would back instead), the presence and strength of competing pro‑independence lists, and how many constituency seats the SNP retains in that region — all data points absent from the provided sources.