THE “significant” tactical voting by supporters of the Union in Holyrood elections has prevented the SNP from winning a majority twice, a new study has found.

The strategy of backing the Unionist candidate most likely to win has also prevented dozens of SNP candidates from winning constituency seats, according to the analysis.

Eoghan Kelly, a PhD candidate at Queen’s University Belfast, looked at the history of strategic voting in all Scottish Parliamentary elections since devolution in 1999, comparing the results of the constituency and regional list ballots.

He found that Labour and Conservative voters are willing to work together in some circumstances to defeat a “significant number of pro-independence candidates” – particularly after the success of the SNP in 2007.

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“Essentially Unionist voters have been very effective in how they use the two ballots, particularly after about 2007/2011,” he said.

“They really understand the difference between the two ballots very clearly and are willing to act in a strategic way.

“It has been very, very successful – far more successful than I was expecting it to be.”

The paper, published in the journal Party Politics, found overall the SNP lost 42 constituency seats despite coming first on the list ballot.

It concludes: “Unionist voters became a more cohesive strategic bloc after 2007. This is contemporaneous with surging support for the SNP so is likely a reaction to that growth.

“Regression analysis finds statistically significant intra-unionist voting to defeat the SNP.

“This intra-unionist voting appears to have been extremely successful, denying them consecutive majorities in both 2016 and 2021 and preventing dozens of SNP candidates from winning constituency seats.”

It highlights Dumbarton as a “prime example” of a constituency where this is happening – finding that strategic voting has impacted on the results four elections in a row.

The paper said: “The results indicate substantial strategic voting with unionist voters rallying behind Labour on the constituency ballot.

“The SNP received 11.8% more votes than Labour on the regional ballot, making them the most popular party on the ballot which does not encourage strategic action.

“However, up to 12.8% percent of voters split their tickets between the Conservatives on the list and Labour’s constituency candidate.

“59% of Conservative list voters appear to have adopted this strategy, which matches estimates from Germany where 50%–58% of voters who had strong incentives to vote strategically actually did so.”

Kelly said: “Similar things have happened in Dumbarton multiple times, it is the place it happens the most. But it has happened across Scotland in quite a few constituencies.”

He also said tactical voting is more likely to have an impact in Scotland than in England, for example.

“In England, when a party wins they win 55/60% of the vote - in Scotland they tend to win under 50% which makes it possible to overtake them,” he said.

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“The chances of actually changing the seat are much higher in Scotland, just because of the way constituencies are drawn.

“That is true at Westminster and at the Scottish Parliament, although the constituencies are different.”

Earlier this year Scottish Tory leader Douglas Ross suggested his party supporters could vote Labour in the next General Election in seats where Keir Starmer’s party had a better chance of winning against the SNP.

In an interview he said unionist voters should consider putting country above party and “do what is best for the country” and support “the strongest candidate to beat the SNP”.

But he later backtracked on the idea, claiming he had “never said” his supporters should vote Labour after the strategy was rejected by Prime Minister Rishi Sunak.

When it comes to how significant strategic voting could be at the next General Election, Kelly said the results from this study could not be used for Westminster because of different constituency boundaries.

He added: “I would imagine there would be some impact, it will be difficult to tease out as it’s easier at the [Scottish] parliament level to see if because there is the two ballots.

“It is hard to tell if it is an honest change in preference say from Conservatives to Labour, than if it is specifically strategic.

“This has always been the issue with strategic voting measures in UK General Elections, in that you have nothing to compare it to.”