Scottish election: How important is the Sun's support?

The Sun is the biggest-selling tabloid newspaper in Scotland - its sales overtook those of its rival the Daily Record nearly five years ago.

It is bought every day by 330,000 Scots and probably read by about three times that number.

There is no doubt that The Sun's decision to back the SNP will put wind in the party's sails.

Historically, many nationalists were perplexed that a party which enjoyed the support of a substantial proportion of Scots was not backed by any mainstream newspapers.

That started to change in 2007 when a number of titles - including the Sunday Herald and the once slavishly Conservative Scottish Sunday Express - offered varying degrees of support to the SNP's Holyrood campaign.

But, apart from being a welcome shot in the arm to a party, how much do newspaper endorsements really matter?

It is exactly 25 years since The Sun started to pay attention to the distinct Scottish market and first seriously challenged the Daily Record for the attention and affection of Scottish tabloid readers.

During that time its political allegiances have changed several times.

The Daily Record supports the Scottish Labour Party

At first it continued to echo the UK Sun's support for the Conservatives and Margaret Thatcher but in 1992 came a big change. The Sun proclaimed its support for independence with the memorable headline: "Rise Now And Be A Nation Again."

Crucially, it backed independence but not the SNP although this subtlety is sometimes forgotten by commentators.

But in 1997 it joined the UK Sun in backing New Labour and Tony Blair.

At the last Holyrood election the Scottish Sun's front page certainly proved controversial. A map of Scotland placed in a hangman's noose shaped like the SNP's logo. More recently, while the UK Sun returned to the Conservative fold the Scottish edition merely withdrew its support for Labour.

So given the symbolic importance of the Sun's support for the SNP - though, ironically given its stand 20 years ago, not independence - what sort of impact might it have on the election?

While few, if any, people would slavishly vote the way their favourite paper tells them, newspaper endorsements can help persuade undecided voters or put momentum behind a party's campaign.

Indeed the extent to which papers influenced voters was always debatable.

Some people buy newspapers which broadly reflect their outlook on the world anyway.

'Won it'

On the other hand, UK-wide research 20 years ago suggested a significant proportion of Sun readers did not actually realise it was a Conservative newspaper despite its keen support for the party and regular attacks on then Labour leader Neil Kinnock.

After the 1992 general election, The Sun claimed it "won it" for John Major after instructing the last person left in Britain after a Labour victory to turn off the lights.

The paper may well have only influenced a few of its readers directly, but in a tight contest a few hundred people in marginal constituencies may have made a real difference.

However since 1992 - and in recent years especially - the sales of many newspapers in Scotland have slipped significantly with readers turning to the web or free titles like Metro.

While The Sun is the top-selling daily paper in Scotland, its circulation has been significantly higher at points in the recent past.

The Sun's endorsement of the SNP can do the party no harm. How much actual good it does them though will not become clear for some time.