MORE often than not, it's been a ritual humiliation. UK party leaders have rarely had it easy when they've headed north to campaign in a Scottish parliament election.

Tony Blair hit the campaign trail twice in 2003, as did Iain Duncan Smith. Both endured the taunts of the Nationalists, who branded them "day-trippers" who would do untold harm their parties' campaigns.

Mr Blair didn't do that, as it happened, as Labour achieved a comfortable victory in the second Scottish parliament election after devolution.

But he did in 2007, when he campaigned in Scotland during the final few weeks of his premiership.

He admitted as much in his autobiography, suggesting Labour would have won the election (it lost to the SNP by a single seat) had Gordon Brown been leader by then.

Mr Brown - despite regular forays into many Holyrood campaigns - never campaigned in one as Labour leader but his successor, Ed Miliband did.

It wasn't a success. When, five years ago, he told Scots that a victory at Holyrood would be a stepping stone for Labour's return to power at Westminster, it made the Scottish parliament sound like a sideshow. The message sank like a stone, and Labour was routed.

Of all UK leaders, perhaps only the late Charles Kennedy really enjoyed the Holyrood campaign trail and was a real asset to his party.

Yet despite this chequered history, they've all come. Even a tricky day on the stump, strategists reckoned, was better than the embarrassment of a no-show.

That calculation has now been turned on its head by the Scots Tories.

During a visit to a buffalo farm in Fife, Ruth Davidson confirmed that David Cameron will be the first UK party leader not to campaign in the final stages of a Holyrood campaign.

The official reason is his hectic schedule. Sources insist a planned visit had to be postponed and an alternative date proved impossible to find.

But that won't quell the speculation he was asked to stay away. As the brickbats started flying, Ms Davidson was quickly accused of putting an "artificial distance" between herself and the Prime Minister.

Whatever the reason, Scots Tory campaign chiefs will not be too dismayed. According the polls, they are tantalisingly close to pulling off a remarkable coup: beating once-dominant Labour into thirds place at Holyrood. They don't want anything to throw Ms Davidson's tour of buffalo farms, ice-cream parlours, mountain tops and race tracks off course.

It will not have escaped their notice that Mr Cameron's standing in the eyes of Scots voters has nosedived following the Panama Papers scandal when, after a string of evasive answers, it was dragged out him that he had owned shares in an offshore fund set up by his late father.

In the past month, his approval rating has plummeted from an already-low minus 35 to a truly dismal minus 46.

More significantly even that that, Mr Cameron embodies the "Tory toff" image that Ms Davidson has worked so hard to move away from.

Since taking over as leader she has championed a working class brand of Conservatism based on graft and self-reliance. The party she wants to present is one for folk with a few quid in RBS, not fortunes salted away in Panama.

During a generally slick campaign, she has struggled the most when called upon to defend Mr Cameron and George Osborne, the Chancellor.

She's looked far more comfortable leading from the front (her own approval rating is minus 11, slightly better than Labour leader Kezia Dugdale) and hammering her main campaign pledges: to defend the Union and stand up the SNP at Holyrood.

The latest poll, by Survation, puts support for Labour on 18 per cent in the constituency vote, a point ahead of the Tories, who enjoy an identical lead in the regional ballot.

On those figures, one seat projection gave Ms Davidson's party 22 seats to Labour's 21, a remarkable result if repeated on May 5.

Overtaking Labour may yet be out of reach for the Scots Tories. Undecided voters would much prefer Labour to come second the SNP. And Labour's pledge to raise taxes to protect public services - which polls suggest is popular - will come under the spotlight when the party launches its manifesto on Wednesday.

Mr Cameron may not be travelling north, but he'll be following the next 10 days as closely as anyone.