THE HOLYROOD election will be dominated by an unprecedented battle for second votes as a fresh poll showed the SNP is set to win almost every constituency in Scotland.

With less than 100 days to polling day, opposition parties look set to rely almost entirely on gaining "top up" list MSPs forcing party leaders into a bitter struggle for support in the regional section of the vote.

It has emerged that the Tories have shifted their focus to regional contests while the Greens yesterday made a direct appeal to SNP supporters.

The nationalists' huge support means Nicola Sturgeon is unlikely to pick up more than a handful of list seats as a result of Holyrood's voting system.

Polls predict Labour will lose all 15 of its constituencies meaning it would be entirely reliant on second votes to return MSPs. It is set to publish the results of its bruising internal fight for regional list slots tomorrow. While lists were once an afterthought for the party, this year's will prove the most important since the Scottish Parliament was established in 1999.

Tory insiders revealed that Ruth Davidson is set to ramp up her 'second vote for the union' strategy, with the Conservatives to pour 80 per cent of resources into a campaign designed to persuade traditional Labour and Lib Dem voters who oppose tax rises and independence to back the party on the list.

The SNP has called on its supporters to resist the temptation to use the second vote tactically. However, smaller pro-independence parties are hoping to exploit a scenario - illustrated by a new TNS poll - which showed that despite Nicola Sturgeon's party enjoying over 50 per cent support in both first and second vote intentions, the result was predicted to translate to the SNP taking 69 of 73 constituencies but only four of 56 regional seats if repeated on election day.

Mr Harvie, setting out his election strategy ahead of his official campaign launch on Monday, made a play for SNP voters as party strategists indicated the Greens have set a target of at least 10 seats, up from two currently, believing they can return an MSP in every region and potentially two in Glasgow and Lothian if its second votes plan proves successful.

He said: "A great many SNP voters, frankly even some of their activists and campaigners, know that a regional vote for the SNP may not be worth very much. If you're in the region where the SNP look likely to take the lion's share or potentially even all constituencies, then your regional vote is going to be divided by a huge number before it's even counted.

"We've got a track record since devolution not just of offering a distinctive policy agenda but of working constructively where we can. Many SNP members who want a bolder land reform agenda, want a full permanent ban on fracking know they'll get progress on those issues, just as we've had progress on issues like rent controls in the private rental sector."

He also dismissed the challenge of RISE, a socialist, pro-independence alliance which emerged following the referendum, saying: "I don't think they are going to make life difficult for us."

RISE is proposing a second referendum to leave the UK by 2021 as part of its drive to win the second votes of SNP supporters inpatient for independence.

Holyrood's regional list seats are assigned through a system of proportional representation which sees the weight of second votes reduce in proportion to success of the party in the constituency section, which is decided on first-past-the-post. John Curtice, seen as the UK's top polling expert, said that on current trends the SNP could expect to win list seats in only three of eight Holyrood regions.

The Tory plan, to sink resources into regional battles rather than constituencies, represents almost a complete reversal of the party's approach to previous Holyrood elections.

Insiders believe that recent Labour and LibDem policy shifts in favour of across-the-board income tax rises represent "a gift" to Ruth Davidson with yesterday's TNS poll - which put the party on 17 per cent in both the constituency and list sections - setting the Tories on course to achieve its highest ever cohort of MSPs.

A senior Tory source added: "We're in the game in a few constituencies, but we'll put a much greater emphasis on the second vote which Ruth will say is an intelligent vote for the union. There are voters who've backed Labour or the LibDems in the past who want a party behind the UK, that will hold the SNP to account and not whack up tax. "We're the only ones who tick those boxes."