PRO-INDEPENDENCE parties should find it easier to demonstrate momentum in next month’s local elections, the country’s leading pollster has said.

Professor Sir John Curtice of Strathclyde University said the Unionist camp was at a “potential disadvantage” as Labour and Tory supporters were reluctant to back each other’s parties through the STV preferential voting system.

SNP and Green supporters were more likely to give each other lower preference votes, giving “the impression that there is momentum behind support for independence”. 

Alex Salmond's Alba is also part of the mix of Yes parties this year.

Prof Curtice makes the point in a new paper on the use of the single transferable vote, which has elected Scotland’s councillors since 2007.

Written for the Electoral Reform Society, the paper found voters have increasingly adapted to the STV, making ever greater use of their preferential votes. 

However Prof Curtice said they were also more polarised, with Yes supporters "less likely to give a lower preference to a unionist candidate", and Unionists "less likely to give a lower preference to a pro-independence candidate," pointing to the most constitutionally divided council elections so far on May 5. 

STV allows voters to rank candidates in order of preference, letting them back more than one party, and next month’s election will be the fourth time voters have headed to the polls using the system.

The research found that in 2017 almost 86% of valid ballot papers had at least two preferences, similar to the figure in 2012 and well up on the 78% that did so in 2007.

In 2017, just over 60% of valid ballot papers contained three or more preferences, up on 55.8% in 2012 and 54% in 2007.

The analysis also found the constitutional question was shaping how voters rank candidates.

Some 46% of SNP voters gave their next preference, after all SNP candidates had been eliminated from the count, to a party other than the Conservatives, Labour, or Liberal Democrats. This was well up on the 18% that did so in 2012.

Just 24% of SNP supporters gave their next preference to one of the three main unionist parties, well down on the 38% who did in 2012.

And Labour backers in 2017 were much more likely than they had been in 2012 to give their next preference to a candidate from another unionist party, either a LibDem, at 26%, or Conservative, 12%. This, in both cases, was around double that in 2012.

Prof Curtice said: “Independence supporters were less likely to give a lower preference to a unionist candidate, while backers of the Union were less likely to give a lower preference to a pro-independence candidate.

“Meanwhile, the pattern of voting behaviour in last year’s Holyrood election suggests that this polarisation of Yes and No supporters may well be even more marked in this year’s local ballot.

“Consequently, the outcome in May is unlikely just to turn on the distribution of first preferences.

“It will also depend on how Yes and No voters use the opportunity afforded by the STV ballot paper to express more than one choice – and on what the parties do or do not do to encourage them to do so.”

Transfers had played a greater role in deciding the eventual winner than before with only 38.5% of candidates elected on first preferences alone in 2017, five points down on the equivalent figure in 2012, and slightly below the 40% who were elected that way in 2007.

In the last local election as many as 101 seats, 8% of the total, were won by candidates who were not initially in a winning position, well up on the 68 seats in 2012 and 73 in 2007.

In 2017, around seven in ten Tory, Labour, and SNP supporters gave preferences to other parties or independents when there were no more candidates of their first-choice party left.

Darren Hughes, chief executive of the Electoral Reform Society, said: “In Scotland, we see an electorate that has embraced this new form of voting, ranking their preferences instead of being forced by a winner takes all system to take a gamble on one option, which they often view as the least worst.

“With local authorities in Wales now also able to make the change to STV, the results in Scotland offer a powerful example of the benefits of adopting a fairer system.

“Where local councils north of the border have led the way it’s time for the rest of the UK to follow and embrace the power of preferences, so making proportional representation the norm.”